Complexes of alkali metal cations with trifluoromethyl:

A computational investigation on the structure and stability

of MC–(CF3) (MZLi, Na, K) isomers

Jun Lianga,b,c,*, Haiyang Lia,c,*, Ying Liua,c, Shuang Chenga,c

aDalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, People’s Republic of ChinabDepartment of Physics, Wuhu Teacher College, Anhui Normal University, Wuhu 241008, People’s Republic of China

cAnhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China

Received 27 January 2005; revised 9 March 2005; accepted 10 March 2005

Available online 13 June 2005

Abstract

The stationary points characterizing the potential energy profiles of the complex process of the MC (MZLi, Na, K) and CF3 were

investigated by B3LYP in conjunction with the 6-311CG(2d,2p) basis set. The optimized geometries, chemical bonding and NBO analysis

indicate that the complexes of MC (MZLi, Na, K) and CF3 exist as ion–dipole molecules. The calculated affinity energies of the most stable

isomer of LiC, NaC, and KC complexes exceed 10.1 kJ/mol, and these values suggest that the MC–CF3 (MZLi, Na, K) complexes could be

observed as stable species in gas phase, which supports Fujii’s proposal that LiC ion attachment mass spectrometry can serve as a

conceivable technique to detect and quantify the emissions of the perfluorocarbons.

q 2005 Elsevier B.V. All rights reserved.

Keywords: Gas-phase chemistry; Trifluoromethyl; Alkali metal ion; Ion attachment mass spectrometry

1. Introduction

In 1971, a new method known as alkali metal ion

attachment chemical ionization mass spectrometry was

proposed by Beauchamp [1] and others [2,3]. In this

method, alkali metal ions such as LiC, NaC, and KC form

adduct ions (also referred to as cationized molecules) with

molecular species through association reactions. In general,

these cationized molecules are more stable than radical

molecular ions or protonated molecules in the gas phase.

The unique features of the LiC–MS system have been

extensively used by Fujii and co-workers [4–10] to detect

intermediate free radicals, novel molecular species, and

unfamiliar and interstellar species in various MW discharge

plasmas. Reactions between alkali metal cations and the

perfluoro-compounds (PFCs) have attracted a great deal of

0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.theochem.2005.03.031

* Corresponding authors. Address: Department of Physics, Wuhu

Teacher College, Anhui Normal University, Wuhu 241008, People’s

Republic of China. Tel.: C86 551 559 3204; fax: C86 553 577 1530.

E-mail address: liangjun@aiofm.ac.cn (J. Liang).

attention in the last two decades, for these reactions are

involved in a significant number of relevant processes in

atmosphere chemistry [11–15]. CF3 is one of the important

radicals in the stratosphere. In recent years, the CF3 free

radical has been the subject of considerable research

interest. This important species is believed to play a

significant role in the ozone layer depletion in the

atmosphere and a variety of ‘down to earth’ processes

such as semiconductor etching and halocarbon fire suppres-

sion [16–18]. In a recent study, Fujii and Arulmozhiraja [11]

have performed density functional theory (DFT) calcu-

lations to investigate the feasibility of measuring the PFCs

in exhaust from semiconductor industry by LiC ion

attachment mass spectrometry. At the B3LYP/6-311CG(3df) level of theory, their computed LiC affinities range

from 12.3 kcal molK1 for CF4 to 21.1 kcal molK1 for C4F8,

and these values are large enough to suggest that industrial

emissions of these PFCs could be in principle detected and

quantified by LiC ion attachment experiments. Recently, we

have investigated the stationary points characterizing the

potential energy profiles of the complex processes of the

MC (MZH, Li, Na, K) and NF3 by Density Functional

Theory (DFT) using the B3LYP hybrid potential and

Journal of Molecular Structure: THEOCHEM 725 (2005) 151–155

www.elsevier.com/locate/theochem

J. Liang et al. / Journal of Molecular Structure: THEOCHEM 725 (2005) 151–155152

the 6-311Cg (2d, 2p) basis set [19]. It was found that the

combinations of MC (MZLi, Na, K) to the F atoms of NF3

may occur in two distinct ways, leading to the formation of

the monocoordinated isomer and the dicoordinated isomer.

Stimulated by these findings, we decided to perform DFT

and ab initio calculations on the structure and stability of the

complexes between the alkali metal cation and trifluor-

omethyl radical. From the applied point of view, it is of

interest to investigate whether the alkali metal cation affinity

of CF3 is large enough to suggest the possible experimental

detection and quantification of CF3 by the alkali metal ion

attachment spectrometry. From the fundamental point of

view, the study of the MC–(CF3) (MZLi, Na, K) isomers is

of interest to learn more on the behavior of CF3 as a gaseous

Lewis base.

Fig. 1. The optimized geometry of the MC (MZLi, Na, K) and CF3

complexes.

2. Computational details

All calculations were performed using GAUSSIAN 03

program [20]. The Becke’s three parameter hybrid density

functional, B3LYP, which includes a mixture of Hartree–

Fock exchange and DFT exchange correlation has been used

[21,22]. The ligands, as well as complexes, were

optimized first at the B3LYP functional using the 6-311CG(2d,2p) basis set followed by the frequency calculation.

The binding energies (DE) were obtained from the

difference between the total energy of the complex

[E(MCKCF3)] and the sum of the total energies of the

corresponding MC (MZLi, Na, K) ion [E(MC)] and

trifluoromethyl [E(CF3)] using the optimized energies:

DEZ ½EðMC � CF3Þ�K f½EðCF3Þ�C ½EðMCÞ�g. These opti-

mized geometries were characterized by harmonic

vibrational frequency calculations at the same B3LYP/

6-311CG(2d,2p) level. These frequency calculations

also yielded the zero-point energies (ZPE),

which were left unscaled; thermal corrections (at

298.15 K) were needed for the calculation of enthalpies.

Binding enthalpies (MC affinities of ligands)

were then calculated using the following relation: DHZDEC DEZPE CDEthermal CDðPVÞ, where D(PV)ZnRTZK0.593 kcal molK1 at 298.15 K. MC (MZLi, Na,

K) affinities were calculated using the energies of the

optimized structures at B3LYP/6-311CG(2d,2p) level and

corrected by ZPE and thermal corrections, which were

obtained at B3LYP/6-311CG(2d,2p) level. Chemical

bonding analysis was based on the theory of Atoms in

Molecules [23], using the implentation in GAUSSIAN 03 due to

Cioslowski and co-workers [24–28]. In particular, for each

structure we have calculated the B3LYP/6-311CG(2d,2p)

charge density r and the Laplacian of the charge density D2r

at the bond critical points (BCP), intended as the points on

the attractor interaction lines where VrZ0. Atomic charges

were derived using the natural population analysis (NPA)

scheme [29,30].

3. Results and discussion

3.1. Geometries and chemical bonding analysis

The connectivities of the various MC–(CF3) (MZLi, Na,

K) isomers presently located as stationary points on the

B3LYP potential energy surfaces, henceforth indicated as

structures 1–4, are shown in Fig. 1. Their detailed

geometries, the results of chemical bonding analysis and

the NBO charges are collected in Tables 1–3, respectively,

For comparative purposes and also to appreciate the

performance of the various employed theoretical levels,

we have also investigated the uncoordinated ligand CF3.

The addition of MC to the C atom of CF3 leads to the

formation of C3V symmetry ion 1, which was characterized

as a local minimum point for LiC and second-order saddle

points for NaC and KC, respectively, on their respective

energy surfaces. In structure 1, r(Li–C), r(Na–C), r(K–C)

are longer than 2.4 A, which are structurally-telling of ion–

dipole adducts between LiC, NaC, KC and CF3. Accord-

ingly, from Table 2, the BCPs on the attractor interaction

lines corresponding to the MC–C bonds are associated with

charge densities r as small as 0.012, 0.008, and 0.004 e/a.u.3

for LiC, NaC and KC, respectively, and with positive

values of the Laplacian of r, which are typical of

electrostatic interactions. Inspection of Table 3, the NBO

charges on the Li, Na, K are larger than 0.95, but charges on

CF3 less than 0.05. LiC, NaC, KC and CF3 are in the two

different charge units, which indicate weak interactions

between MC (MZLi, Na, K) and CF3. Combining NBO

charge distribution and the above bond length discussion,

MC and CF3 complexes would exist as MC–CF3(MZLi,

Na, K) ion–dipole type complexes. The optimized structure

Table 1

The geometry parameters of the optimized isomers of the MC (MZLi, Na,

K) and CF3 complexes

M Li Na K

Structure1

M–C 2.4994 2.9349 3.5134

C–F 1.3088 1.3119 1.3152

M–C–F 105.25 105.7798 106.3075

F–C–F 113.34 112.8968 112.4411

Structure 2

M–F1 1.828 2.2298 2.6839

C–F 1.2886 1.2975 1.3032

C–F1 1.4248 1.3907 1.3723

F–C–F 115.0422 114.0658 113.4684

F–C–F1 108.5486 109.3388 109.818

M–F1–C 160.1175 176.8916 177.7107

Structure 3

M–F 2.0813 2.4891 2.9515

C–F 1.359 1.3488 1.3419

C–F1 1.2787 1.2877 1.2947

F–C–F 104.76 106.7559 107.9931

F–C–F1 112.945 112.4981 112.1789

Structure 4

M–F 2.4777 2.83199 3.33385

C–F 1.3298 1.3275 1.32600

F–C–F 108.7782 109.5025 110.0746

M–F–C 79.9017 70.5601 38.0466

CF3

C–F 1.3222

F–C–F 111.3562

Bond lengths in angstroms (A), bond angles in degrees (8).

J. Liang et al. / Journal of Molecular Structure: THEOCHEM 725 (2005) 151–155 153

1 and the corresponding bonding analysis reveal also that

the formal attachment of MC to the C atom of CF3 enhances

backdonation from the surrounding fluorines. Thus, at the

B3LYP level of theory, the C–F distances of structure 1 are

shorter than uncoordinated CF3 by ca. 0.013, 0.010 and

0.007 A, respectively, and the F–C–F bond angle are larger

by ca. 1.98, 1.66 and 1.118. Consistently, passing from

uncoordinated CF3 to structure 1, the charge densities of the

C–F bonds slightly increase and corresponding Laplacian

become more negative.

Table 2

B3LYP/6-311C(2d,2p) charge densities r (e/a.u.3) and Laplacian of the charge de

the attractor interaction lines of the MCKCF3 (MZLi, Na, K) complexes 1–4 an

Species Li Na

BCP r V2r BCP r

CF3 C–F 0.29165 K0.3101 C–F 0.

Structure 1 C–F 0.30453 K0.3380 C–F 0.

Li–C 0.01163 C0.0411 Na–C 0.

Structure 2 C–F 0.31860 K0.2551 C–F 0.

C–F1 0.22026 K0.3359 C–F1 0.

Li–F1 0.02901 C0.2450 Na–F1 0.

Structure 3 C–F 0.26630 K0.3770 C–F 0.

C–F1 0.32622 K0.2124 C–F1 0.

Li–F 0.16312 C0.1114 Na–F 0.

Structure 4 C–F 0.28778 K0.3394 C–F 0.

Li–C 0.00587 C0.0405 Na–F 0.

Searching for the complexes arising from the ligation of

MC to the fluorine atoms of CF3 leads to the location of

three distinct structures, namely, the monocoordinated ion 2

of Cs symmetry, the dicoordinated ion 3 of Cs symmetry,

and the tricoordinated ion 4 of C3V symmetry. However, at

the B3LYP/6-311CG(2d,2p) level of theory, only isomers 2

and 3 revealed as true minima on the potential energy

surface, whereas ion 4 revealed second-order saddle points,

unstable with respect to the degenerate bending motion of

the alkali metal atoms. Generally speaking, we first wish to

note that, although the ligation of the alkali metal cations to

the fluorine atom(s) of CF3 leads to sometimes appreciable

structural changes with respect to the uncoordinated ligand,

the BCPs on the attractor interaction lines corresponding to

the M–F (MZLi, Na, K) bonds of structures 2, 3, and 4

are invariably characterized by low values of the

charge densities and positive values of the corresponding

Laplacian, thus suggesting the formation of essentially

electrostatic complexes between MC (MZLi, Na, K) and

CF3. From Table 3, NBO analysis indicates that LiC, NaC,

KC and CF3 are in the two different charge units, and the

positive charges are almost on LiC, NaC, and KC,

respectively, which also indicate that LiC, NaC, KC and

CF3 complexes existing as MC–CF3 (MZLi, Na, K) ion–

dipole type complexes.

Structure 2 is the most stable structure in the four isomers

on their potential surfaces, respectively, and they are global

minima. The optimized structures of structure 2 reveal that

the ligation of MC (MZLi, Na, K) to a single F atom of CF3

induces an appreciable elongation of the involved C–F

bond. Thus, from Table 1, we note that, at the B3LYP level

of theory, the C–F1 bond distance of structure 2 is larger

than the C–F distance of CF3 by ca. 0.10, 0.08, and 0.05 A

for LiCKCF3, NaCKCF3, KCKCF3, respectively. In

addition, the C–F bond distances of structure 2 are

invariably predicted to be shorter than CF3 by ca. 0.033,

0.024, 0.019 A, and the F–C–F bond angle result larger by

ca. 3.70, 2.73 and 2.138 for LiC–CF3, NaC–CF3, KC–CF3,

respectively. From Table 2, all these structural changes

nsities V2r (e/a.u.5), evaluated at the bond critical points (BCP) located on

d CF3 (for connectivities and labeling of the atoms, see Fig. 1)

K

V2r BCP r V2r

29165 K0.3101 C–F 0.29165 K0.3101

30140 K0.3292 C–F 0.29823 K0.3217

00751 C0.0269 K–C 0.00443 C0.0135

31123 K0.2685 C–F 0.30674 K0.2794

24080 K0.3575 C–F1 0.25366 K0.3547

01900 C0.1356 K–F1 0.01335 C0.0700

27261 K0.3559 C–F 0.27719 K0.3420

31880 K0.2360 C–F1 0.31321 K0.2535

11342 C0.0677 K–F 0.00810 C0.0398

28869 K0.3261 C–F 0.28935 K0.3199

00480 C0.0290 K–F 0.00321 C0.0174

Table 3

NBO charges for the equilibrium geometries of the MC (MZ Li, Na, K)

and CF3 complexes

MCKCF3 Structure 1 Structure 2 Structure 3 Structure 4

LiCKCF3

C 0.91761 1.07615 1.07485 1.07461

Li 0.95041 0.99211 0.97360 0.96918

F K0.28934 K0.50149 K0.26339 K0.34793

F K0.28934 K0.28339 K0.39253 K0.34793

F K0.28934 K0.28339 K0.39253 K0.34793

NaCKCF3

C 0.95152 1.06567 1.06352 1.06864

Na 0.95778 0.99439 0.98143 0.97680

F K0.30310 K0.45998 K0.28053 K0.34848

F K0.30310 K0.30004 K0.38221 K0.34848

F K0.30310 K0.30004 K0.38221 K0.34848

KCKCF3

C 0.96340 1.05197 1.05193 1.05756

K 0.98312 0.99578 0.99080 0.98881

F K0.31551 K0.42903 K0.29482 K0.34879

F K0.31551 K0.30936 K0.37396 K0.34879

F K0.31551 K0.30936 K0.37396 K0.34879

NPA atomic charges.

Table 4

The total energies (E), zero-point energies (ZPE) and relative energies (DE)

of the optimized geometries of the MC (MZLi, Na, K) and CF3 complexes

M Li Na K

MCCCF3

E K344.948227 K499.750835 K937.424357

ZPE 0.011833 0.011833 0.011833

DE 0.0 0.0 0.0

Structure 1

E K344.952098 K499.752690 K937.423475

ZPE 0.012781 0.012283 0.012101

DE K10.16 K7.52 2.32

Structure 2

E K344.967558 K499.761318 K937.429266

ZPE 0.012610 0.012232 0.012038

DE K50.75 K27.52 K12.89

Structure 3

E K344.965135 K499.760155 K937.428301

ZPE 0.012821 0.012270 0.012091

DE K44.39 K24.47 K10.35

Structure 4

E K344.954862 K499.755306 K937.426220

ZPE 0.012196 0.012015 0.011905

DE K17.42 K12.26 K4.89

The total energies (E), zero-point energies (ZPE) in hartree. Relative

energies (DE) in kJ/mol.

J. Liang et al. / Journal of Molecular Structure: THEOCHEM 725 (2005) 151–155154

parallel the differences computed in the charge densities of

the C–F bonds of CF3 and structure 2.

The location of the dicoordinated isomer 3 as a true

energy minimum on the potential energy surface provides

the first indication that, with selected electrophiles,

trifluoromethyl may actually behave as a ‘bidentate’

gaseous base. The comparison between the optimized

structure of isomer 3 and uncoordinated CF3 confirms the

expected elongation of the two C–F bonds involved in the

direct interaction with the alkali metal cations. These

structural changes result, however, less pronounced than

structure 2. Consistently, the C–F1 bond lengths result

shorter than CF3 by ca. 0.044, 0.034, 0.028 A for LiC–CF3,

NaC–CF3, KC–CF3, respectively. Once again, compared

with CF3, the charge densities of the C–F bonds of structure

3 are in line with these computed structural changes.

As already pointed out, the tricoordinated structure 4

revealed second-order saddle points on their respective

potential energy surfaces, and their optimized geometries

suggest a rather weak interactions between MC (MZLi,

Na, K) and CF3. Thus, the C–F bond length is appreciably

larger than the same parameter of structures 2 and 3, and the

structural features and charge densities of the CF3 moieties

in the MC–CF3 complexes are only slightly different with

respect to uncoordinated CF3.

3.2. Energies

The relative stabilities of the presently investigated MC–

CF3 (MZLi, Na, K) complexes, obtained at different levels

of theory, are reported in Table 4. The most important

general indication from these calculations is that, irrespec-

tive of the employed basis set and the selected level of

theory, the fluorine-coordinated isomers 2, 3, and 4 are

invariably predicted to be more stable than the carbon-

coordinated isomer 1. In addition, the tricoordinated isomer

4 is invariably predicted as the least stable among the

fluorine-coordinated structures.

One of the aims of the present study was to investigate

the possibility that trifluoromethyl could be detected by MC

(MZLi, Na, K) ion spectrometry. The gas-phase MC cation

affinity (CA) of CF3, namely the minus enthalpy change of

the reaction.

MC CCF3 /MC–ðCF3Þ

From data in Table 4, the affinity energies of various LiC,

NaC, KC complexes are calculated. The CAs of most stable

structure 2 of LiC, NaC, KC complexes are 50.7, 27.5,

12.9 kJ/mol. The affinity energies are 44.4, 24.5, 10.4 kJ/

mol for local minima structure 3, respectively. We see that

the CA energies follow the order LiCONaCOKC in the

four structures 1–4, and the affinity energies of the most

stable isomers exceed 10.1 kJ/mol, which are large enough

to suggest that CF3 could be in principle detected and

quantified by LiC, NaC, KC ion attachment experiments,

and this supports the Fujii’s [8] proposal that LiC ion

attachment mass spectrometry as a conceivable technique to

detect and quantify the PFCs.

Acknowledgements

This work was supported by the National Natural Science

Foundation and the Director Research Grants of Hefei

J. Liang et al. / Journal of Molecular Structure: THEOCHEM 725 (2005) 151–155 155

Institute of Physical Sciences and Anhui Institute of Optics

and Fine Mechanics, Chinese Academy of Science.

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