Does cation break the cyano bond? A critical evaluation of nitrile-cation interaction

ORIGINAL PAPER

Does cation break the cyano bond? A critical evaluationof nitrile-cation interaction

Pei Meng Woi & Maizathul Akmam A. Bakar & Ahmad Nazmi Rosli &Vannajan Sanghiran Lee & Mohd Rais Ahmad & Sharifuddin Md Zain &

Yatimah Alias

Received: 28 December 2013 /Accepted: 23 March 2014 /Published online: 27 April 2014# Springer-Verlag Berlin Heidelberg 2014

Abstract DFT and G4 results reveal that cations displaythe following trends in imparting its positive charge toacrylonitrile; H+>Li+>Na+>K+ for group I and Be2+>Mg2+>Ca2+ for group II. Solvation by water moleculesand interaction with cation make the cyano bond morepolarized and exhibits ketene-imine character. Bond or-der in nitrile-cation complexes has been predicted basedon the s character of the covalent bond orbitals. Mulli-ken, CHELPG, and NPA charges are in good agreementin predicting positive charge buildup and GIAO nucleardeshileding on C1. G4 enthalpies show that Mg2+ ismore strongly bound to acrylonitrile than to acetonitrileby 3 kcal mol−1, and the proton affinity of the former ishigher by 0.8 kcal mol−1. G4 enthalpies of reductionssupport prior experimental observation that metalatedconjugated nitriles show enhanced reactivity towardweak nucleophiles to afford Michael addition products.

Keywords Bond order prediction . B3LYP optimizedstructures . Ketene-imine . Cyano bond . G4 enthalpies .

π-conjugated nitrile

Introduction

Metal ions play an essential role in enzymes functions [1].Metal participation in enzymatic reactions increases the rate ofchemical transformations via a wide range of mechanisticpathways – enhancing nucleophilicity, acidity, and electrophi-licity of reacting species, promoting heterolysis of substratesand acting as reduction-oxidation centers in redox reactions[2]. Interaction between nitriles and metal cations (iron, cobaltand nickel) has been proposed in nitrile metabolisms involv-ing nitrile hydratase (NHase; EC 4.2.1.84) and nitrileaminohydrolase (EC 3.5.5.1) [3–8]. In addition, these en-zymes have been employed as catalysts in preparative organicchemistry [9–11] and as resolution agent to separate racemicmixture [12]. On the other hand, Group I (Li+, Na+, K+) andgroup II cations (Mg2+ and Ca2+) are frequently encounteredin biological systems as ionophores – lipid-soluble moleculesthat facilitate transport of mobile ions across hydrophobicmembrane or hydrophilic pores (channels) that allow cationsto pass through [13–16]. Naturally occurring polyether anti-biotics have widely been used for group I and II cationrecognitions; ionomycin (Ca2+), monensin (Na+), nonactin(NH4

+), calcimycin (A23187, Ca2+) and valinomycin (K+)[17]. There has been a growing interest to investigate theinteraction between group I and II metal ions and neutralmolecules for chemical sensing purposes [18]. Donor-acceptor type interaction between lone electron pairs in het-eroatoms (i.e., oxygen, nitrogen and sulfur) and cations hasfrequently been suggested in explaining preferential binding[19]. Consequently, in chemical sensor systems, selectivitytoward target analyte presumably is determined by the bindingenergies between metal ions and neutral receptors [20, 21].

We are interested in using computational methods to eval-uate electron-donor functional groups such as nitriles, aminesand carbonyls as cation recognition agents. The cyano func-tionality is known to be stable and resistant toward chemical

Electronic supplementary material The online version of this article(doi:10.1007/s00894-014-2219-3) contains supplementary material,which is available to authorized users.

P. M. Woi (*) :M. A. A. Bakar :A. N. Rosli :V. S. Lee :S. Md Zain :Y. AliasDepartment of Chemistry, Faculty of Science Building, University ofMalaya, 50603 Kuala Lumpur, Malaysiae-mail: [email protected]

M. R. AhmadNEMS & Photonics Laboratory, MIMOS Berhad, Technology ParkMalaysia, Kuala Lumpur 57000, Malaysia

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http://dx.doi.org/10.1007/s00894-014-2219-3

attacks, i.e., only strong reducing agents reduce it to primaryamine. Due to its ease of preparation [22] and stability inpolymeric membranes [23, 24] we explore the possibility ofusing carbonitriles for sensing applications. We have recentlyreported rational design of chemical sensor array using un-conjugated carbonitriles [25]. We have demonstrated experi-mentally in the report that lipophilic carbonitriles form ho-mogenous mixtures with typically employed polymer matri-ces and dopants, and the resulting sensing membranes detectcation analytes. In this work, we investigate the possibility ofpreferential bindings between group I and II cations withconjugated carbonitriles.We adopt the DFTmethod in gaininginsights on how the lone-electron pair on the nitrogen atom ofthe cyano group interacts with cation, and how the resultingcomplex distributes the received positive charge through a π-conjugation network (Scheme 1). Acrylonitrile has been cho-sen as a conjugated carbonitrile receptor model due to com-putational feasibility. Geometry optimization, frequency anal-ysis, bond order calculation, magnetic shielding analysis, andsolvation effect study on this simple model using DFT andother programs in Gaussian09 could be completed withinhours.

Numerous prior findings have shown that polar solventsform strong interaction with metal cations and solvation in-fluence the nature of cation-analyte binding activity [26–31].Therefore, in analyzing preferential binding interactions be-tween cations and acrylonitrile receptor model, and in eluci-dating structural changes in acrylonitrile due to cation bindingusing the B3LYP density functional theory [32–35], we ac-count for the solvent effect by including the polarized contin-uum model (PCM) into our calculations [36]. Positive chargebuildup on carbon nuclei and migration of electron density arededuced from magnetic shielding coefficients using GIAOapproach [37]. Bond orbital occupancies and percentages of2s versus 2p contributions on σ-bonds are obtained usingNBO program [38]. Prediction on bond orders in order toelucidate the efficiency in overlaps between bonding orbitalsare analyzed using atoms-in-molecule (AIM) approach [39,40]. Thermochemical argument on the difference betweensaturated and π-conjugated nitriles is supported by G4 en-thalpies of dissociations, reduction reactions and gas phaseacidities [41].

Results and discussion

The one to one complexes between acrylonitrile and group Iand II cations (H+, Li+, Na+, K+, Be2+, Mg2+, and Ca2+) wereoptimized with Becke-3-parameter (B3) for exchange part[32], the Lee–Yang–Parr (LYP) for correlation function [33],and 6-31+G(d,p) basis sets [42, 43] for non-metals andAhlrich’s qzvp basis set [44, 45] for metals. The optimizedgeometries and the structural changes due to cation bindingswere analyzed in terms of Mulliken, natural and electrostaticcharges [46], bond distances, bond angles, natural bond orbitalpopulations [38], vibrational frequencies [47], and nuclearmagnetic shieldings [37].

Structural analysis

The optimized structures of acrylonitrile (Fig. 1) and its com-plexes with group I: H+, Li+, Na+, K+, (Fig. 2) and group II:Be2+, Mg2+ and Ca2+ (Fig. 3) are shown below. Table 1 showsthe bond distances of acrylonitrile and its complexes withgroup I and II cations obtained from the optimized geometriesin vacuum and water medium. In the gas phase, interactionsbetween group II cations and the cyano lone-electron pair onthe nitrogen atom elongate the C-N bond distances by 1.9 %,0.9 %, and 0.9 % in the complexes with Be2+, Mg2+, and Ca2+

respectively. However, the interactions with the group I cat-ions result in mostly negligible changes; except for the inter-actions with H+ where the C3-N bond distance is reduced by0.9 %. On the contrary, the interactions in water slightlyreduce the C3-N bond distances, and again except for hydro-gen ion that shows 1.2 % reduction in bond length. Suchoccurrence presumably is due to stronger binding interactionsbetween group II cations and the nitrogen atom on the cyanogroup. Moreover, while the interactions result in the extensionof the bond distance in vacuum, these interactions lead toshorter C3-N bond distances in water medium. The observa-tion can be explained in terms of solvation of the metal cationsby water molecules that results in weaker interactions of themetal cations with the cyano nitrogen atom in water medium.

Fig. 1 Optimized geometries of acrylonitrile (A) calculated usingB3LYP with 6-31+G(d,p) in (i) the gas phase and (ii) water mediumScheme 1 Transfer of charge from cation to acrylonitrile (A) π-network

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Table 1 also shows that in the gas phase binding with Be2+

causes the largest (3.6 %) shortening of the C2-C3 bonddistance, whereas both Na+ and K+ cause the least effect (bothgive rise to 0.3 % and 0.1 % reduction respectively). The restof the cations cause the C2-C3 bond distances to shrink by0.6 % to 2.1 %. On the contrary, in water medium, theshrinking of C2-C3 bond distances caused by all cations areless significant compared to that in the gas phase. Again, Be2+

causes the largest reduction (1.6 %), whereas, the C2-C3reduction caused by Li+, Na+, and K+ are negligible (0.1 %).

In striking contrast to the effect of cation binding on theC2-C3 bond length, the C1-C2 bond elongates due to interac-tion with the analytes. In vacuum, the largest elongation of theC1-C2 bond distance is caused by Be2+ (2.2 %), whereas Na+

and K+ do not give any noticeable difference (both increase by0.1 %). The other cationic analytes cause increase of the C1-C2 bond length by 0.2% to 1.1 %. In water medium, the watermolecules surround the positively charged analytes and sig-nificantly reduce the interactions between the cations andacrylonitrile. In Be2+, H+, and Mg2+ complexes, only slight

(i) Acrylonitrile-H+ + (water)

(iii) Acrylonitrile-Li+ + (water)

(v) Acrylonitrile-Na+ + (water)

(vii) Acrylonitrile-K+ + (water)

(iii) Acrylonitrile-Mg2+

(vac) (ii) Acrylonitrile-H

(vac) (iv) Acrylonitrile-Li

(vac) (vi) Acrylonitrile-Na

(vac) (viii) Acrylonitrile-K

(vac) (iv) Acrylonitrile-Mg2+ (water)

Fig. 2 Optimized geometries of acrylonitrile complexes with group I(H+, Li+, Na+, and K+) calculated using B3LYP with 6-31+G(d,p) basisset for non-metals and qzvp basis set for metals, in the gas phase and inwater medium (iefpcm model). (i) Acrylonitrile-H+ (vac) (ii) Acryloni-trile-H+ (water) (iii) Acrylonitrile-Li+ (vac) (iv) Acrylonitrile-Li+ (water)(v) Acrylonitrile-Na+ (vac) (vi) Acrylonitrile-Na+ (water) (vii) Acryloni-trile-K+(vac) (viii) Acrylonitrile-K+ (water)

(i) Acrylonitrile-Be2+ (vac) (ii) Acrylonitrile-Be2+ (water)

(iii) Acrylonitrile-Mg2+ (vac) (iv) Acrylonitrile-Mg2+ (water)

(v) Acrylonitrile-Ca2+ (vac) (vi) Acrylonitrile-Ca2+ (water)

Fig. 3 Optimized geometries of acrylonitrile complexes with group II(Be2+, Mg2+, and Ca2+), calculated using B3LYP with 6-31+G(d,p) basisset for non-metals and qzvp basis set for metals, in the gas phase and inwater medium (iefpcm model). (i) Acrylonitrile-Be2+ (vac) (ii)Acrylonitrile-Be2+ (water) (iii) Acrylonitrile-Mg2+ (vac) (iv)Acrylonitrile-Mg2+ (water) (v) Acrylonitrile-Ca2+ (vac) (vi)Acrylonitrile-Ca2+ (water)

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increase of the C1-C2 bond distances have been observed –0.4 % increase in Be2+ and H+ complexes and 0.1 % in Mg2+

complex.The observed changes of bond distances in the C2-C3 and

C1-C2 bonds are expected to change the respective bondorders according to Scheme 1. When the positively chargedanalyte binds with the lone-electron pair of the cyano group itfirst imparts some positive charge on the nitrogen atom. Sinceacrylonitrile has a vinyl group directly bonded to the cyanocarbon, the positive charge on nitrogen is distributed further toC1 – thus the positive charge from the analyte is nowdelocalized over the entire π-network. Moreover, the resultsin Table 1 can be rationalized in terms of the ionic radii of theanalytes, i.e., analytes with smaller ionic radii result in shorterbond distances; group I - H+<Li+<Na+<K+ and group II-Be2+<Mg2+<Ca2+. The observed structural changes due tocation binding can also be explained in terms of dissociationenergy of the complexes (Table 2); shorter N-M bond dis-tances in the complexes correspond to higher dissociationenergies. Furthermore, the results indicate that the interactionof metal cations to acrylonitrile do not affect all the C-H bondlengths.

The C1-C2-C3, H5-C1-H6, C2-C3-N and C3-N-M bondangles can potentially be affected by interaction with cations(Table 3). The C1-C2-C3 bond angles are significantly affect-ed by binding with metal cations and hydrogen ion.With Be2+

and H+ analytes the C1-C2-C3 bond angle is reduced by 3degrees (in vacuum), suggesting large extent ofrehybridization of the C2 carbon. Similar to the observationin bond lengths, in water medium the solvation by watermolecules causes weaker interaction between cations andacrylonitrile - thus results in less significant changes. Like-wise, the ionic radii and charge to radius ratios argument isapplicable. Ca2+ and K+ having the least charge to radiusratios result in the least noticeable changes. In contrast to thedecrease in C1-C2-C3 angle due to cation binding, the chang-es in the H5-C1-H6 and C2-C3-N bond angles are negligible.

The C2-C3-N bond angles in the gas phase and water mediumremain at about 178°, and the H5-C1-H6 angle is unchangedat about 117°.

Atomic charge distribution

In providing rationale for preferential binding between acry-lonitrile and cations, charge transfer mechanism and redistri-bution of donated charge over the entire π-network is a keyelement in structural analysis. Mechanism in Scheme 1 sug-gests that after binding with a cation the cyano nitrogenbecomes formally a positive charge center, and the residualcharge on the cation gives rise to repulsive force between thetwo atoms and this does not promote binding. In conjugatedcarbonitriles, the unfavorable repulsive force could be miti-gated as the positive charge on nitrogen is delocalized througha π- network (Scheme 1).

Table 1 Bond distances (Å) ofacrylonitrile (A) and its com-plexes with group I (H+, Li+, Na+,and K+) and group II cations(Be2+, Mg2+, and Ca2+), calculat-ed using B3LYP with 6-31+G(d,p) basis set for non-metalsand qzvp basis set for metals, inthe gas phase and in water medi-um (iefpcm model)

Structure C1-C2 BD C2-C3 BD C3-N BD N-M BD

Vac H2O Vac H2O Vac H2O Vac H2O

Group I

A-H+ 1.349 1.346 1.410 1.411 1.154 1.151 1.008 1.008

A-Li+ 1.344 1.342 1.424 1.429 1.164 1.162 1.885 2.086

A-Na+ 1.343 1.343 1.427 1.430 1.164 1.164 2.260 2.433

A-K+ 1.342 1.342 1.430 1.431 1.164 1.164 2.433 2.905

Group II

A-Be2+ 1.370 1.347 1.380 1.409 1.186 1.161 1.484 1.588

A-Mg2+ 1.356 1.343 1.403 1.423 1.174 1.161 1.919 2.072

A-Ca2+ 1.353 1.342 1.407 1.427 1.175 1.162 2.200 2.482

A 1.341 1.341 1.432 1.432 1.164 1.165 NA NA

Table 2 Dissociation energies (De) of acrylonitrile complexes with groupI (H+, Li+, Na+, and K+) and group II cations (Be2+, Mg2+, and Ca2+),calculated using B3LYP with 6-31+G (d,p) basis set for non-metals andqzvp basis set for metals, in the gas phase and in water medium (iefpcmmodel)

Structure De (kcal mol-1)

Vac H2O

Group I

CH2=CHCN-H+ → CH2=CHCN+H+ 192.0 140.2

CH2=CHCN-Li+ → CH2=CHCN+Li+ 45.1 2.5

CH2=CHCN-Na+ → CH2=CHCN+Na+ 32.5 2.6

CH2=CHCN-K+ → CH2=CHCN+K+ 22.6 1.9

Group II

CH2=CHCN-Be2+ → CH2=CHCN+Be2+ 206.6 45.0

CH2=CHCN-Mg2+ → CH2=CHCN+Mg2+ 119.3 11.7

CH2=CHCN-Ca2+ → CH2=CHCN+Ca2+ 88.2 5.4


Table 4 shows a general trend in positive charge donatingstrength on group I; H+>Li+>Na+ with 0.372, 0.708, 0.949 ofMulliken charges remaining on the cations, respectively. Ingroup II, beryllium, having the highest charge to radius ratioshows the highest donating capability. Be2+ donates 60.2 % ofits Mulliken charge and only retains 0.797 of its charge [46].As predicted based on its charge to radius ratio, Mg2+ retains1.468 of its Mulliken charge. Electrostatic potential method(CHELPG) and natural population (NPA) data show similartrend, giving comparable values, but significantly differentfrom Mulliken charges.

Again, the same trends are observed in water medium.Mulliken charge data in Table 4 show that interaction withcation causes positive charge build up on C1 and N, whereas,C3 appears to be more negatively charged in the complexes.Numerous reports have highlighted basis set dependency ofatomic charges based onMulliken population, and the chargesbeing quite different compared to those from other methods[48].

Therefore, the atomic charges in acrylonitrile and its com-plex with H+, Li+, Na+, Be2+, and Mg2+ have been examinedusing electrostatic (CHELPG) and natural population (NPA)approaches for a more reliable charge distribution (Fig. 4) [48,49]. All three methods are in good agreement on C1 charge –the three methods predict a positive charge buildup on thiscarbon due to interaction with cation. In striking contrast toMulliken charges, the CHELPG and NPA methods showsignificant positive charge buildup on C3 and very highnegative charge buildup on N due to cation binding. More-over, the CHELPG and NPA methods show that Mg2+ onlydonates 7.7 % and 4.05 % of its positive charge to acryloni-trile, respectively.

Our data confirm earlier reports that the Mulliken chargesare quite different from the electrostatic and natural charges. Inorder to compare the three methods of charge analysis further,

we relate the predicted charges to the observed NMR chemicalshifts of C1, based on GIAO method (Fig. 5). The plot showsthat the three methods are consistent in predicting the C1nuclear deshieldings due to charge buildup, but the slopesare different.

Orbital occupancy analysis

Orbital occupancy analysis can help elucidate the structure ofacrylonitrile receptor model, its reactivity toward electrophilesand the relative stability of the acrylonitrile-cation complexes[38]. In addition, the NBO analysis also provides the relativeimportance of s and p orbitals (s versus p character) in formingchemical bonds or interactions of interest. The bond distancesin uncomplexed acrylonitrile can illustrate this point. Theexperimental C3-N bond in acrylonitrile (1.164 Å) is longerthan those in hydrogen cyanide (1.156 Å) and acetonitrile(1.157 Å) [50]. This striking difference can be rationalizedin terms of overlapping of neighboring 2pz orbitals in C2 andC3 atoms (p-p conjugation) in acrylonitrile that causes elon-gation of the C3-N bond (Scheme 2). The π-conjugationnetwork is illustrated in the HOMO-3 MO of acrylonitrileand is shown in Fig. 6(ii). Furthermore, Fig. 6(i) suggests thatthe HOMO-2 MO of the receptor model is used to formbinding interaction between the nitrogen lone electron pairand the cations. Moreover, Fig. 6(iii) indicates that HOMO-3acrylonitrile-Mg2+ complex is involved in the stabilizing π-conjugation network. For illustrative purposes, we focus theNBO analysis on the complexes of Mg2+ and Na+. The totalorbital occupancies in C1 and C2 atoms are significantlygreater than the valence electron for carbon atom (4). Anincrease of 8.5 % is mainly contributed by hydrogen atomsbonded to these carbons. On the contrary, the total occupancyof C3 is reduced by 7.8 %, while that of N atom is increased

Table 3 The bond angles of ac-rylonitrile (A) and its complexeswith group I (H+, Li+, Na+, andK+) and group II cations (Be2+,Mg2+, and Ca2+), calculated usingB3LYP with 6-31+G (d,p) basisset for non-metals and qzvp basisset for metals, in the gas phaseand in water medium (iefpcmmodel)

Structure Bond angles (°)

C1-C2-C3 H5-C1-H6 C2-C3-N C3-N-Mn+


Group I

A-H+ 120.8 120.7 117.6 118.0 178.4 178.6 179.9 179.5

A-Li+ 121.8 122.3 117.5 117.8 178.3 178.4 179.1 173.5

A -Na+ 122.0 122.3 117.5 117.8 178.4 178.3 178.8 176.5

A-K+ 122.3 122.4 117.5 117.8 178.3 178.4 178.1 177.0

Group II

A-Be2+ 122.0 120.8 117.7 118.1 177.0 178.5 178.5 179.2

A-Mg2+ 121.8 121.6 117.4 117.9 177.4 178.3 178.0 178.5

A-Ca2+ 121.8 122.3 117.4 117.9 177.6 177.9 178.3 170.4

A 123.0 122.5 117.7 117.7 178.5 178.5 NA NA


by only 5.6 %. In forming the cyano bond, the more electro-negative N atom withdraws electron density from C3. How-ever, since N atom does not receive all the electrons donatedby C3, it is presumed that C2 receives the balance 2.2 %.

From the orbital occupancies in Table 5 it can be deducedthat C1 and C2 use 2s, 2px and 2py orbitals for constructingthe sp2 hybrid orbitals for bonding to H atoms and each other,and 2pz to form the C1-C2 π-bond. Likewise, C3 and N use 2sand 2px orbital to make the sp hybrid orbitals for bonding toeach other and to C2, and for forming a lone electron pair onN. The 2py and 2pz orbitals are used to make two cyano π-bonds.

Solvation by water molecules causes apparent changes inthe total orbital occupancies – C1 and C3 show reduced

occupancies of 0.6 % and 0.8 %, respectively, C2 receivesadditional population by 0.4 % and N experiences a very largeincrease of 2.4 % in population [51–53]. Furthermore, inwater medium there are significant changes in relative contri-butions of individual orbitals. Solvation by water significantlyreduces the occupancies of the 2px orbitals in C3 and N,whereas, the occupancies of the 2s, 2py, and 2pz in N aresignificantly increased. This observation suggests that hydra-tion significantly changes the chemistry of the cyano func-tional group; it exhibits ketene-imine characteristics. The C3-N bond appears to bemore polarized in water, and the increasein electron populations in the 2s, 2py, and 2pz of the N atomscan be rationalized in terms of enlarging the N lone electronpair, the 2py, and 2pz orbitals in order to make the 2p orbitals

Table 4 Analysis of charges onC1, C2, C3, N, and M+ for acry-lonitrile (A) and its complexeswith group I (H+, Li+, and Na+ )and group II cations (Be2+ andMg2+ ), calculated using B3LYPwith 6-31+G (d,p) basis set fornon-metals and qzvp basis set formetals, in (a) gas phase and in (b)water medium (iefpcm model)

Charge Method Group I Group II A

A-H+ A-Li+ A-Na+ A-Be2+ A-Mg2+

a) Gas phase

C1 Mulliken −0.170 −0.212 −0.211 −0.091 −0.149 −0.250CHELPG 0.012 −0.058 −0.079 0.245 0.108 −0.199NPA −0.186 −0.257 −0.273 −0.057 −0.143 −0.351

C2 Mulliken 0.278 0.001 0.157 0.319 0.131 0.173

CHELPG −0.306 −0.326 −0.300 −0.526 −1.454 −0.193NPA −0.433 −0.407 −0.400 −0.455 −0.452 0.352

C3 Mulliken 0.169 −0.068 −0.055 −0.013 −0.157 0.071

CHELPG 0.646 0.714 0.649 1.105 0.951 0.461

NPA 0.645 0.481 0.432 0.761 −0.646 0.250

N Mulliken −0.336 −0.036 −0.427 0.177 −0.023 −0.492CHELPG −0.447 −0.788 −0.721 −1.161 −1.105 −0.494NPA −0.429 −0.613 −0.558 −0.981 −0.876 −0.300

M Mulliken 0.372 0.708 0.949 0.797 1.468 NACHELPG 0.524 0.946 0.954 1.701 1.847

NPA 0.526 0.969 0.986 1.777 1.919

b) Water medium

C1 Mulliken −0.187 −0.245 −0.245 −0.179 −0.223 −0.256CHELPG −0.019 −0.121 −0.158 0.019 −0.074 −0.156NPA -.0208 −0.305 −0.312 −0.208 −0.269 −0.325

C2 Mulliken 0.271 −0.046 0.007 0.200 0.003 0.177

CHELPG −0.288 −0.250 −0.241 −0.400 −0.323 −0.247NPA −0.421 −0.386 −0.383 −0.421 −0.402 −0.376

C3 Mulliken 0.180 0.025 0.015 0.030 −0.080 0.114

CHELPG 0.660 0.581 0.549 0.917 0.737 0.535

NPA 0.646 0.389 0.361 0.648 0.494 0.300

N Mulliken −0.323 −0.157 −0.405 0.028 0.002 −0.598CHELPG −0.441 −0.647 −0.612 −0.919 −0.779 −0.598NPA −0.413 −0.485 −0.457 −0.780 −0.625 −0.387

M Mulliken 0.384 0.841 0.971 1.244 1.682 NACHELPG 0.533 0.968 0.970 1.829 1.924

NPA 0.532 0.980 0.993 1.898 1.973


in N pointed away in space, perpendicular to each other andfacilitate formation of H-bonds with water molecules in the x,y, and z directions (Scheme 2).

Interactions with Mg2+ in the gas phase causes dramaticchanges on the total occupancies in C3 and N (Table 6). Thevalence electron population in C3 is reduced by 10.5 % (from3.72 to 3.33), whereas, the occupancy of N increased by 9.8%(from 5.28 to 5.8). The changes in C1 and C2 are alsosignificant; the occupancy in C1 reduced by 4.8 % and thatof C2 increased by 2.3 %. The C1 atom almost experiences ahalf-empty 2pz orbital, suggesting a positive charge buildupon this nuclei, and C1 has transferred its π-electrons to C2.Consequently, the 2pz orbital in C2 experiences an increase inoccupancy by 6.8 % - contributing the total occupancy of 4.44in C2, among the highest valence for carbon atom seen in thiswork. On the contrary, the total occupancy of 3.33 in C3 isamong the lowest occupancy for carbon atom observed in thiswork.

The changes in occupancies in C2 and C3 can be related tothe CHELPG charges observed earlier, i.e., increase in C2population results in negative charge buildup and decrease inC3 occupancy corresponds to positive charge buildup on C3(Fig. 4). The 2px orbital in C3, having occupancy close to 1(0.977), appears to retain its electron used for forming σ-bonds with C2 and N, whereas, the occupancies in 2py and2pz dramatically reduced to only 0.71 and 0.78, respectively –thus polarizing the C3-N cyano bond to almost having aformal negative charge on N (supported by the CHELPG data;N having occupancies of 1.34 and 1.41 for 2py and 2pz,respectively). Solvation bywater molecules causes weakeningof the binding between Mg2+ and acrylonitrile and migrationof the π-electron density from N and C2 to C1 and C3.Furthermore, at individual orbital level, hydration restoresthe occupancies of the 2pz orbitals in C1 and C3 to about0.8, very similar to the hydrated uncomplexed acrylonitrile.

The effect of Na+ to orbital occupancies in acrylonitrile isless apparent compared to those described earlier for Mg2+.However, similar trends of total orbital occupancies for eachcarbon and nitrogen atom and relative contribution by indi-vidual orbitals are observed in both cases. It is noteworthy thathydration causes less significant changes in the Na+ complexwith acrylonitrile, compared to the Mg2+ complex describedearlier. The less noticeable effects caused by solvation of Na+

Fig. 4 CHELPG electrostatic charge of acrylonitrile (i) and its com-plexes with Mg2+(ii), Na+(iii) and H+(iv)

Fig. 5 Comparison of Mulliken, CHELPG and NPA charges by relatingit to GIAO NMR shift

Scheme 2 C3-N bond polarization due to (i) hydration and (ii) cationbinding


by water molecules further suggests that the interaction be-tween Na+ and acrylonitrile has been weaker to begin with(compared to the interaction with Mg2+) (Table 7).

The C1-C2 σ-bond has significantly higher s character thanthe ideal 33.3 % predicted for sp2 hybridization. In the gasphase C1 and C2 contribute 38.1 % and 39.5 % to the C1-C2σ-bond, respectively, and this suggests that the hydrogen andcarbon atoms bonded to C1 and C2 are polarized, i.e., havingsignificant positive charges on its nuclei and facile deproton-ation of vinylic protons in the gas phase (Supplementary data)[54] and for preparative purposes [55] have been reportedearlier. Binding with Mg2+ causes reduction of 4.5 % (at C1)in the s character of the C1-C2 σ-bond. The interaction withMg2+ causes positive charge buildup on C1 due to migrationof electron from C1 to C2 through π-conjugation. The

positively charged C1 tends to make the H atoms bonded toit more negatively charged (σ-conjugation), thus reducing thes contribution to the sp2 hybrid orbitals. Likewise, since theinteraction between Na+ and acrylonitrile is weaker than thatin Mg2+ complex, Na+ only causes 1.6 % reduction to the scharacter contribution to C1-C2 σ-bond (at C1). Hydrationcauses an increase of the s character in the C1-C2 σ-bond by2.4 % and 4.8 %, at C1 and C2 respectively. Solvation bywater molecules make the polarized C-H bonds more stableand further increase the ability of H atoms to hold positivecharge and contribute even more s character to the C1-C2 σ-bond.

From Table 8, the C2-C3 σ-bond shows an extreme con-tributions from the two carbon atoms, in using s orbitals forforming a σ-bond. While C2 only uses 29.8 % of its 2s tomake the C2-C3 σ-bond in the gas phase, C3 contributes52.6 % of s character for this bond, larger than the ideal50 % for sp hybrid orbitals. A plausible explanation for thisobservation lies on the bonding requirement for the adjacentatoms – C2 has used a high s character (39.5 %) in bondingwith C1 and presumably N does not need a high s character informing the C3-N cyano bond. The C2 carbon atom can beregarded as a sp2 center while the C3 can be considered as sphybridized. On the other hand the C2-C3 in acrylonitrile isusually viewed as a carbon-carbon single bond. If an sp2

orbital fromC2 and an sp orbital fromC3 are used to constructthe C2-C3 σ-bond, the resulting bond would be too short,

(i) Acrylonitrile HOMO-2 (ii) Acrylonitrile HOMO-3

(iii) Acrylonitrile –Mg2+

HOMO-3(iv) Acrylonitrile –Mg2+

HOMO-4

Fig. 6 Molecular orbitals of acrylonitrile and its complex with Mg2+ inwater. (i) Acrylonitrile HOMO-2 (ii) Acrylonitrile HOMO-3 (iii) Acrylo-nitrile –Mg2+ HOMO-3 (iv) Acrylonitrile –Mg2+ HOMO-4

Table 5 Orbital occupancies of acrylonitrile

Atom Medium 2s 2px 2py 2pz Total

C1 Vac 1.076 1.159 1.189 0.915 4.339

H2O 1.082 1.181 1.193 0.855 4.311

C2 Vac 1.001 1.062 1.227 1.050 4.340

H2O 1.030 1.021 1.236 1.070 4.357

C3 Vac 0.863 0.999 0.924 0.931 3.717

H2O 1.086 0.915 0.834 0.851 3.686

N Vac 1.596 1.473 1.118 1.092 5.279

H2O 1.774 1.244 1.182 1.216 5.416

Table 6 Orbital occupancies of acrylonitrile-Mg2+ ion complex


C1 Vac 1.11 1.189 1.166 0.667 4.132

H2O 1.088 1.185 1.170 0.814 4.257

C2 Vac 1.017 1.034 1.267 1.122 4.440

H2O 1.009 1.044 1.249 1.088 4.390

C3 Vac 0.867 0.977 0.711 0.776 3.331

H2O 0.857 0.987 0.807 0.833 3.484

N Vac 1.514 1.580 1.337 1.414 5.845

H2O 1.541 1.572 1.222 1.253 5.588

Table 7 Orbital occupancies of acrylonitrile-Na+ complexd


C1 Vac 1.086 1.187 1.167 0.821 4.261

H2O 1.083 1.183 1.172 0.861 4.299

C2 Vac 1.067 1.048 1.244 1.090 4.449

H2O 1.066 1.053 1.240 1.072 4.431

C3 Vac 0.863 0.991 0.832 0.854 3.540

H2O 0.863 0.994 0.860 0.885 3.602

N Vac 1.567 1.547 1.193 1.224 5.531

H2O 1.579 1.530 1.155 1.169 5.433


unacceptable for a single bond. Therefore, it can be expectedthat C2 and C3 would optimize their contributions so that bothwould satisfy their neighbors (C1 and N) while maintaining ansp3-like and single C2-C3 σ-bond. In keeping with this argu-ment, C2 and C3 contribute rather extreme proportions of scharacter of the C2-C3 σ-bond - only 28.6 % from C2 and55.8 % from C3 (in vacuum). Hydration by water moleculescauses the biggest change on the occupancies in the C3-Ncyano bond. Similar to the widely accepted explanation forenzymatic hydrolysis of nitriles [3–5], water molecules pre-sumably approach both the C3 and N terminals.

At N lone-electron pair and 2p orbitals interact inhydrogen-bond fashion with water, whereas C3 becomeselectrophilic (ketene-imine like) and attracts oxygen atomsof water as soft nucleophile (Scheme 2). Hydration signifi-cantly reduces the contribution of s character to the C2-C3 σ-bond to only 26.1 %, even closer to an ideal sp3 hybrid orbital.Likewise, the interaction between acrylonitrile and Mg2+ re-sults in similar changes seen in water solvation. In addition,Mg2+ increases the total s character from C2 and C3(combined) by 2.4 % (from 82.4 to 84.4). The increase inthe combined s character of the C2-C3 bond indicates that inthe optimized structure of the Mg2+ complex the proposedelectron migrates from C1-C2 bond to C2-C3 bond(Scheme 1). Furthermore, binding with Na+ also reduces thes contribution from C2 and increases that from C3. However,

there is no apparent change in the combined s character (from82.4 to 82.8).

The C3-N bond uses close to equal contributions of 2s and2p orbitals in forming the cyano σ-bond –C3 has 47.5%s and52.5 % p, while N uses 44.8 %s and 54.8 % p characters,resulting in almost ideal sp hybrid orbitals. Solvation by watermolecules dramatically reduces the s contribution in N to only21.4 % - less than that used for sp3 bond. This can beexplained in terms of elongation of the C3-N bond, reducingof the cyano bond order, formation of the N-H hydrogenbonds, and increase of metalated ketene-imine characteristics.Moreover, binding with Mg2+ causes reduction in the p char-acter in N (and increase in s character), presumably promotingthe interaction between N and Mg2+.

2.4 13C NMR shielding and vibrational frequency analysis

The NMR [37] results show that the C1 and C3 nuclei inacrylonitrile have NMR shifts of 122.3 and 99.2 ppm, respec-tively. These are consistent with sp2 and sp hybridized centersfor C1 and C3, respectively, and in good agreement with the103.8 ppm shift reported earlier for acetonitrile, using thesame computational method [25]. Our model for hydratedacrylonitrile suggests significant structural changes causedby solvation of water molecules, based on the widely acceptedview on enzymatic hydrolysis of nitriles [3–5]. From thispoint of view, the cyano bond is elongated, becomes ketene-imine in nature, the C3 experiences positive charge buildupand becomes electrophilic due to solvation by water. Inkeeping with this model, Table 9 shows that in water

Table 8 Percentages of s and p characters in C1-C2, C2-C3 and C3-N4σ-bonds in acrylonitrile (A) and its complexes with Mg2+ and Na+

C1-C2 Medium C1s% C2s% C1p% C2p%

A Vac 38.1 39.5 61.8 60.5

H2O 39.0 41.4 61.0 58.5

A-Mg2+ Vac 36.4 39.0 63.6 61.0

H2O 37.4 39.38 62.5 60.6

A-Na+ Vac 37.5 39.4 62.4 60.6

H2O 37.7 39.4 62.2 60.6

C2-C3 C2s% C3s% C2p% C3p%

A Vac 29.8 52.6 70.1 47.3

H2O 26.1 51.9 73.9 48.1

A-Mg2+ Vac 28.6 55.8 71.3 44.2

H2O 28.63 54.6 71.3 45.4

A-Na+ Vac 28.8 54.0 71.1 45.9

H2O 29.1 53.6 70.9 46.4

C3-N4 C3s% Ns% C3p% Np%

A Vac 47.5 44.8 52.5 54.8

H2O 47.93 21.4 52.0 78.5

A-Mg2+ Vac 44.3 55.4 55.7 44.6

H2O 45.4 52.9 54.5 46.8

A-Na+ Vac 46.0 49.9 54.0 49.8

H2O 46.4 48.2 53.6 51.5

Table 9 Calculated GIAO 13C NMR shifts (ppm, TMS reference) foracrylonitrile (A) and its complexes with group I (H+, Li+, Na+, and K+)and group II cations (Be2+, Mg2+, and Ca2+), calculated using B3LYPwith 6-31+G(d,p) basis set for non-metals and qzvp basis set for metals,in the gas phase and in water medium (iefpcm model)

Structure 13C C1 NMR shift(TMS ref)

13C C3 NMR shift(TMS ref)

Vac H2O Vac H2O

Group I

A-H+ 161.43 155.73 93.85 91.68

A-Li+ 143.49 133.42 107.03 102.87

A-Na+ 139.49 132.13 107.80 104.12

A-K+ 136.34 131.07 104.86 103.12

Group II

A-Be2+ 199.81 155.98 140.70 110.43

A-Mg2+ 172.95 141.14 127.09 105.62

A-Ca2+ 160.69 134.93 118.61 103.36

A 122.26 129.25 99.24 103.49


medium the chemical shifts for C1 and C3 are significantlyincreased by 5.7 % and 5.3 %, respectively.

Binding with group II cations results in enormous increasein the C1 chemical shifts. Be2+ for example causes the C1 shiftto increase dramatically by 63.4 %. The increase of the C1shift due to interactions withMg2+ and Ca2+ are still very largeand as a whole group II shows the Be2+>Mg2+>Ca2+ trend inincreasing the C1 shift– suggesting the ability of the doublycharged cations in donating its positive charge to acrylonitrile.The effect of group I on the increase in the C1 chemical shift ismuch less - about 15 % and the difference within the group Ications are also less noticeable, except for hydrogen ion thatgives comparable effect to those of group II. Therefore, theresults suggest that it is more appropriate to consider H+ as aspecial case.

Cation bindings causes less dramatic changes to the C3chemical shift. While group II cations increase the C3 shiftsby 19.6 % (Be2+) to 41.8 % (Ca2+), the group I cation onlyincrease the C3 shift by about 6 %, and there is hardly anydifference caused by different group I cations. Again, theeffect of hydrogen ion should be treated differently becauseit reduces the C3 chemical shift by about 5 %.

Since our model predicts that binding with cation is ac-companied by changes in atomic charges of interest, it isinstructive to see a relationship of these parameters for allgroup I and II cations. Interestingly, Figs. 6(ii) and 7(i) showsthat the 13C1 shifts and Mulliken charge for all the speciesinvestigated in this work, inclusive of the uncomplexed acry-lonitrile in vacuum and in water medium fit into a single linewith greater than 0.9 correlation coefficient. Moreover, theplot also show apparent trends and relative magnitudes ac-cording to groups and medium. For example, the doublycharged cations show two distinct groups in vacuum and inwater media, and the separations between Be2+ and Mg2+ inboth media are quite apparent. Parameter relationships onacrylonitrile and its complexes with group I and II cations,in vacuum and in water medium are provided in Fig. 7 andin supplementary data †. All these plots are in good accordwith the proposed model (Schemes 1 and 2) and rationale.In order to visualize clusters formed by three parametersconcerning C1 and C3, along with group information incolors (group I, group II and acrylonitrile), three dimensionalplots like Fig. 8 could be plotted to provide additionalinsights.

The experimental bond length of the C3-N cyano bond inacrylonitrile (1.164 Å) is significantly longer than those inacetonitrile (1.157 Å) and hydrogen cyanide (1.156 Å) [50].On the contrary, the C2-C3 bond in acrylonitrile (1.426 Å) ismuch shorter compared to the carbon-carbon bonds in aceto-nitrile (1.458 Å) and ethane (1.536 Å) [50]. Furthermore, theC1-C2 bond in acrylonitrile is identical to the carbon-carbondouble bond in ethylene (1.339 Å) [50]. It is interesting to useexperimental and calculated vibrational frequencies to

elucidate the structure and valence electron population in asimple model of conjugated carbonitrile (acrylonitrile) [47].

Experimental data shows that the stretching frequency ofthe cyano bond in acrylonitrile (2239 cm−1) is indeed lower by1.2 % than that in acetonitrile (2267 cm−1).

Our computational results are in good accord with experi-mental observation – the cyano stretching frequency by DFTmethod gives a 1.2 % lower in acrylonitrile (2335 cm−1) thanin acetonitrile (2364 cm−1) (refers to Table 10). Althoughexperimental data show identical bond length for the vinylbond in acrylonitrile and that in ethylene, actual measurementindicates a slightly lower stretching frequency in acrylonitrile(1615 cm−1) compared to that in ethylene (1623 cm−1) [50].

While experimental data show a significantly shorter C2-C3 bond in acrylonitrile, the calculated stretching frequenciesindicate quite the opposite – the stretching frequency in acry-lonitrile (884 cm−1) is 4.7 % lower compared to that inacetonitrile (929 cm−1). A shorter bond distance would implymore efficient overlap between 2p orbitals that would increasebond order and barrier to rotation [56–58]. For example, theshorter C2-C3 bond in 1,3-butadiene has been viewed ashaving multiple-bond character due to high s character of thehybrid orbitals and π-conjugation [59].

Solvation by water molecules results in small changes inthe C3-N and C1-C2 stretching frequencies and no change inthe C2-C3 frequency. Water molecules stabilize a more polar-ized cyano bond and induce migration of electron fromC1-C2bond to C2-C3 bond. The observed decreases in the stretchingfrequencies of these bonds – 1674 cm−1 (0.4 % reduction) and2322 cm−1 (0.6 % reduction) for C1-C2 and C3-N, respec-tively are consistent with the water solvated model(Scheme 2(ii)).

Binding with Mg2+ causes significant changes in thestretching frequencies of the three bonds of interest; the C-C1and the C3-N stretching frequencies are reduced by 3.6 %(1620 cm−1) and 2.7 % (2271 cm−1) , respectively, whereas,the C2-C3 frequency is significantly increased by 7.9 %(954 cm−1). Interaction between Na+ and acrylonitrile gives riseto noticeably different changes in vibrational frequencies, seenearlier on the Mg2+ complex. Na+ Does not change the C3-Nstretching frequency (2335 cm−1), slightly reduce the C1-C2frequency by 0.7 % (1669 cm−1) and significantly increase theC2-C3 frequency by 1.8 % (900 cm−1). Be2+ Causes the largestreduction to the C3-N stretching frequency (4.5 %, 2229 cm−1).Moreover, Be2+ also causes very large reduction on the C1-C2and C2-C3 stretching frequencies to 1588 cm−1 (5.5 %) and817 cm−1 (7.6 %), respectively, suggesting elongation of allthree bonds of interest due to interaction with Be2+. It isnoteworthy to highlight that Be2+ does not follow thegeneral model provided in Scheme 2(ii). In water mediumthe binding between cations and acrylonitrile is weakened –thus resulting in less significant effects on the stretchingfrequencies in the three bonds of interest.


Mechanistic insights

Binding with cation apparently causes positive charge buildupon C1 (Fig. 4). Group II cations result in higher increase in C1positive charge compared to group I. 13C1 NMR confirms thatdeshielding of the C1 nuclei happens due to cation binding

(Fig. 7(ii)). Scheme 1 suggests that electron density from theC1-C2 bond migrates to C2-C3 giving rise to reduced bondorder in C1-C2 and positive charge buildup on C1. Elongationof the C1-C2 due to this reason is observed in Fig. 7(iii). Thetrend of cation effect on C1 is best described in terms ofcharge to radius ratio and positive charge donating capability

Fig. 7 Parameter relationships for acrylonitrile and its complexes withgroup I (H+, Li+, Na+ and K+) and group II cations (Be2+, Mg2+ andCa2+), calculated using B3LYP with 6-31+G (d,p) basis set for non-metals and qzvp basis set for metals, in the gas phase and in watermedium (iefpcm model). Atomic charges are from Mulliken population.

(i) C1 charge vs. C2-C3 Bond length (ii) 13C1 shift vs. C1 charge (iii) C1Charge vs. C1-C2 bond length (iv) C1 Charge vs. Cation charge to radiusratio (v) C1 charge vs. C3-N bond length (vi) C1 charge vs. Cationdonated charge


of the cations (Fig. 7(iv) and (vi)). Cations with strong positivecharge donating capability (group II) cause elongation of theC3-N bond. Similar to the effect of water solvation, strongelectron acceptors pull electron density from the lone electronpair and the N 2py and 2pz orbitals resulting in highly polar-ized cyano bond with bond order significantly less than 3(Fig. 7(v) and Scheme 2). In contrast to the obvious parameter

relationships on the C1-C2 bond (Fig. 7), some questionsregarding the C2-C3 and C3-N bonds, before and after inter-action with cations, are less obvious; does cation bindingcause positive charge buildup on C3, can the C2-C3-N bondangle deviate significantly from linearity, i.e., to form a cycliccomplex structure? Additional results are provided in supple-mentary data† in order to further elucidate structural changeson acrylonitrile due to cation binding.

For C1-C2 bond order, ethylene, acrylonitrile and proton-ated acrylonitrile were employed and Eq. 1 was derived withgood linearity (R2>0.99). C1-C2 Bond orders of 1.67 and1.78 were obtained for the complexes of acrylonitrile withMg2+ and Na+, respectively. The results suggest significantelongation of the C1-C2 bond lengths due to binding with thetwo cations. Wiberg bond index has been calculated indepen-dently for the C1-C2 in acrylonitrile-Mg2+ complex, and thismethod shows a reduction of 9 % and thus in good agreementwith our prediction based on C1 s character.

C1−C2BondOrder ¼ 0:0927�%C1 scharacter–1:7023½ � ð1Þ

The C2-C3 bond can be viewed as a single carbon-carbonbond. It is interesting to examine the contribution of 2s and2px orbitals in forming the σ-bond. Bond orbital occupancydata show that the C2-C3 σ-bond is constructed from twodifferent types or hybrid orbital; sp2.4 from C2 and sp0.8 fromC3 (supplementary data†). Likewise, 1,3-butadiene, acryloni-trile and protonated acrylonitrile were used in order to relatethe percentage of C2 s character from NBO and the C2-C3bond order obtained from the AIM procedure (Eq. 2, R=0.999) (supplementary data †).

Equation 2 was employed to predict the C2-C3 bond ordersin acrylonitrile complexes with Mg2+ and Na+, and bondorders of 1.18 and 1.14 were obtained, respectively. NBO data

Fig. 8 Three dimensional parameter relationships; (i) C1 Mullikencharge, C1-C2 bond distance and 13C1 NMR shift and (ii) C3 s character,C3-N bond distance and 13C3 NMR shift

Table 10 Vibrational frequenciesfor acrylonitrile and its complexeswith group I (H+, Li+, Na+, andK+) and group II cations (Be2+,Mg2+, and Ca2+), calculated usingB3LYP with 6-31+G(d,p) basisset for non-metals and qzvp basisset for metals, in the gas phaseand in water medium (iefpcmmodel)

Structure Vibrational frequencies (cm−1)

N-M C3-N C2-C3 C1-C2


Group I

A-H+ 3714.39 3715.04 2347.80 2352.85 898.74 899.51 1681.09 1647.87

A-Li+ 502.39 361.68 2338.29 2343.90 915.59 897.86 1664.00 1672.06

A-Na+ 288.36 183.03 2335.06 2336.72 900.40 892.35 1669.42 1672.65

A-K+ 165.48 113.12 2329.82 2329.50 898.36 888.19 1672.21 1673.04

Group II

A-Be2+ 1246.02 722.68 2229.36 2344.53 816.80 1004.12 1587.84 1647.76

A-Mg2+ 463.41 364.57 2270.68 2359.18 954.22 915.53 1620.48 1665.72

A-Ca2+ 332.62 197.85 2278.15 2342.23 923.75 900.23 1639.37 1671.72

A NA NA 2335.49 2321.74 884.49 883.60 1681.09 1673.92


in Table I suggests that binding with stronger positive chargedonor (Mg2+) causes the C2-C3 bond order to increase signif-icantly but with reduction in s character of the C2 orbital. Thissuggests that shorter C2-C3 bond accompanies the increase inbond order but this not achieve through higher C2 s characterbut presumably through more efficient overlaps of 2pz orbitals(π-bond).

C2−C3BondOrder ¼ −0:0368�%C2 scharacter þ 2:2008½ � 2ð Þð2Þ

The C2-C3 bond order in acrylonitrile-Mg2+ complex,computed using Wiberg bond index method shows an in-crease of 11.8 %, consistent with an increase in ketene-iminecharacter. Our previous report suggests that the cyano triplebond in acetonitrile is not elongated due to interaction withcations – there is evidence that suggests that it is shortened insome cases [25]. In striking contrast to the C-N bond inacetonitrile, data shows that cation binding apparently de-creases the bond order of the C3-N bond, and simultaneouslyincreases the C3 s character. In building the relationshipbetween C3 s character and the C3-N bond order, hydrogencyanide, acrylonitrile and protonated acrylonitrile wereemployed, and Eq. 3 with correlation coefficient greater than0.999 was linearly plotted;

C3−N Bond Order ¼ 0:1114�%C3 s character−2:9249½ � ð3Þ

Equation 3 was utilized to predict the C3-N bond orders inacrylonitrile complexes with Mg2+ and Na+, and predictedbond orders of 2.01 and 2.2 were deduced for the complexes,respectively. The predicted cyano bond order reduction be-tween metal cations having different positive charge donatingcapability (Mg2+ vs. Na+) suggests that cation binding couldbe used in organic transformation involving π-conjugatednitriles. The model in Scheme 2 suggests that binding withstrong electron acceptor polarizes the cyano bond in conju-gated carbonitriles and makes the carbon terminus more elec-trophilic and reactive toward relatively mild reagents. Like-wise, employing the Wiberg bond index method, interactionwith Mg2+ causes the C3-N bond to decrease in bond order by12.6 %, thus exhibiting metalated ketene-imine characteristics(Supplementary data).

Computational section

All the calculations were performed using the Becke-3-parameter (B3) for exchange part [32] and the Lee–Yang–Parr(LYP) correlation function [33], with 6-31+G(d,p) for non-metals and qzvp basis set [44, 45] for metals, in vacuum andwater phase (PCM) for the ground state optimization using theGaussian 09 W.C01 suite of program [60]. Gaussian NBOversion 3.1 has been utilized to calculate atomic orbital

occupancies and its contribution to bonding interaction anddelocalization of electron density within the acrylonitrile andmetal ion complexes. The 13C NMR isotropic shielding wascalculated with the GIAO method using the optimized struc-tures obtained from DFT calculations. Gaussian-4 theory is acomposite method wherein a sequence of ab initio calcula-tions is performed in order to obtain a total energy for amolecule of interest [41, 61, 62]. It is the fourth in a seriesof Gn methods whereby the geometry optimization and zero-point energies are done using B3LYP DFT theory with 6-31G(2df,p) basis set and 6-31G(2 fg) for third-row non-transition metal elements. Correlation level calculations arecomputed with Moller-Plesset perturbation theory up tofourth-order and with coupled cluster theory. Correlation cal-culations are done with large basis sets; G3LargeXP basisused in the MP2 (FU) step and, quadrupole-zeta andquintuple-zeta used for extrapolation to the Hartree-Focklimit.

Conclusions

Cation binding does not change the cyano bond distance insaturated carbonitriles. On the contrary, interaction of cationswith conjugated nitriles causes distribution of positive chargeover the entire π-network, and simultaneously elongates theC1-C2 and C3-N bonds. The C2-C3 bond rehybridizes due tocation binding wherein C2 uses orbitals with increasing pcharacter. However, the C2-C3 bond is shortened, presumablydue to more efficient overlap between 2p orbitals and migra-tion of electron density from C1-C2. Orbital occupancies,Mulliken, CHELPG and NPA charges, magnetic shieldings,vibrational frequencies and predicted bond orders stronglysuggest positive charge buildup on C1 and ketene-iminestructure on C2-C3-N [63]. Cations pull electron density fromthe N lone-electron pair and the 2py and 2pz orbitals resultingin weaker cyano π-bonds and imine-like metalated C3-Ndouble bond. Covalent bond s-character has successfully beenused in predicting the bond orders in nitrile-cation complexes.G4 enthalpies show that conjugated nitriles are more reactive,have stronger binding energies with cations and higher protonaffinity. Experimental evidence suggests that cation-complexed conjugated nitriles exhibit enhanced reactivityand react with weak nucleophiles to afford Michael additionproducts. Geometry optimization with 6-31+G(d,p) basis setsshows that the influence of charge transferred from cations toacrylonitrile follows the general trend of Be2+>Mg2+>Ca2+

for group II and H+>Li+>Na+>K+ for group I. Hydrationcauses weakening of cation positive charge donating capabil-ity and poorer interactions between cations and the cyanogroup. B3LYP dissociation energies show that the interactionsbetween acrylonitrile and group II metal cations are strongerthan group I except for H+.


Acknowledgments This work is fully funded by the High ImpactResearch Grant UM-MOE UM.C/625/1/HIR/MOE/F00004-21001 andUMRG Sub-program RP006B-13SUS grants. The authors would like tothankMinistry of Education (MOE) andMinistry of Science, Technologyand Innovation (MOSTI), Malaysia for their support and Farhatun NajatMaluin for proofreading of this article.

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