Solvent Effects on Chemical Exchange in a Push–Pull Ethylene as Studied by NMR and Electronic Structure Calculations

Published: May 31, 2011

r 2011 American Chemical Society 7531 dx.doi.org/10.1021/jp201885q | J. Phys. Chem. A 2011, 115, 7531–7537

ARTICLE

pubs.acs.org/JPCA

Solvent Effects on Chemical Exchange in a Push�Pull Ethylene asStudied by NMR and Electronic Structure CalculationsMaysoon Ababneh-Khasawneh,† Blythe E. Fortier-McGill,‡ Marzia E. Occhionorelli, and Alex D. Bain*

Department of Chemistry, McMaster University, 1280 Main St. West, Hamilton, Ontario, Canada L8S 4M1

bS Supporting Information

’ INTRODUCTION

Much of chemistry happens in solution, so the interactions ofsolutes with their solvent is critical in the understanding andcontrol of chemical reactions.1,2 These interactions are driven byGibbs free energy, which means that both enthalpy and entropycan be important. They are also very complicated interactions, soaccurate experimental data are needed to test hypotheses aboutthe role of solvents. A goodway of probing these interactions is tosee how the rates of chemical reactions change if the solvent ischanged. Because the rates of reactions are extremely sensitive tothe enthalpy and entropy of activation, we can measure thedifference in thermodynamic properties between the ground andtransition states quite accurately. These measurements give ussolid data to compare with our models and calculations.

NMR studies of chemical exchange provide excellent data onreaction rates.3�12 The reaction occurs at the molecular level, butthe overall sample remains the same and measurements cancontinue for as long as the system is stable. Since the reaction rateis governed by the free energy of activation, it is necessary tomeasure rates over a set of temperatures in order to separateenthalpy and entropy effects. The temperature range should be aswide as possible, to give a wide array of rates. NMR methods areavailable that scan rates over several orders of magnitude, to

provide excellent thermodynamic data.13,14 Finally, there aremany molecules that exhibit chemical exchange, so a selection ofchemical probes of solvent effects is available.

One such class of probe molecules is the push�pullethylenes,15�32 but the concept of a push�pull system also goeswell beyond ethylenes.33�43 These are molecules containingmultiple bonds with electron-withdrawing groups on one endand electron-donating groups on the other. Under normalcircumstances, rotation around multiple bonds is rarely seenbecause it involves breaking a formal chemical bond. Thepush�pull perturbation of the electronic structure significantlyweakens the double-bond character, to the extent that restrictedrotation around the nominal carbon�carbon double bond canreadily be observed by NMR. Many push�pull ethylenes havebeen synthesized and studied.4,15,18,33 Moreover, many of themshow substantial entropies of activation, which can be anindicator of strong solvent effects.44�49 The rates are determinedby a combination of the energies of the isolated molecule and itsinteraction with the solvent.

Received: February 26, 2011Revised: May 30, 2011

ABSTRACT: NMR measurements of chemical exchange in a push�pullethylene, dissolved in a number of different solvents, are presented. Theseare complemented by high-level electronic structure calculations, using bothgas-phase conditions and those which simulate solvents. The results showthat it is essential to include entropy effects in order to understand theobserved trends. For instance, the equilibrium state in this case representsthe state with lowest Gibbs free energy, as it must, but not the lowest enthalpy. The particular molecule is methyl 3-dimethylamino-2-cyanocrotonate (MDACC). The geometry at the carbon�carbon double bond can be either E or Z with roughly equalpopulations at ambient temperature.We havemeasured the equilibrium constant and the rates for the exchange between these statesin a number of solvents: methanol, chloroform, acetonitrile, toluene, dichloromethane, acetone, and tetrahydrofuran. Furthermore,theN,N-dimethylamino group attached to the double bond also shows restricted rotation, and this has been measured in both the Eand Z conformations. The equilibrium constant and the three rotational barriers provide excellent probes of the solvent effects.Electronic structure calculations with a number of basis sets up to the 6-311þþG(2df,2p) level, using both Hartree�Fock anddensity functional (B3LYP)methods were used to predict the E and Z ground states, and the three transition states. The calculationswere done for an isolated molecule and also for solvent models representing toluene, acetone, and ethanol. The E conformation ismore stable in solution, is the structure in the crystal, and is also the prediction for the gas phase from the calculations. However, thedependence of the equilibrium constant on temperature shows that the Z conformation actually has lower enthalpy. The stability ofthe E conformation in solution must be due to entropic effects. Similarly, the solvent effect on the E-Z barrier is primarily due toentropy. The measured enthalpy of activation is similar in all the solvents, but the entropy of activation increases with the solventpolarity. The barrier to rotation of the N,N-dimethylamino group shows a combination of entropy and enthalpy effects. Thiscombination of experiments and theory gives an extraordinarily detailed picture of solvent�solute interactions.

7532 dx.doi.org/10.1021/jp201885q |J. Phys. Chem. A 2011, 115, 7531–7537

The Journal of Physical Chemistry A ARTICLE

Electronic structure calculations play a key role in the study ofthese systems.18,23,27�31,50,51 The molecules are small enoughthat high-quality calculations can be done and the details of theelectronic structure, such as the full chemical shielding tensors,52

can be probed. Furthermore, techniques are now available tocalculate solvent effects53�56 in a reliable fashion.

A particularly useful molecular system is methyl 3-dimethyla-mino-2-cyanocrotonate (MDACC) (Figure 1),52,57�59 whichwas chosen as our system to study. This molecule, (also knownas crotonic acid, 2-cyano-3-(dimethyl amino)-, methyl ester, or asmethyl (2E)-2-cyano-3-(dimethylamino)but-2-enoate (in its Econformation)) and its ethyl ester analogue60 (referred to as1-cyano-1-carbethoxy-2-methyl-2-dimethylaminoethylene) havefurther dynamic processes (Scheme 1) beyond the rotationabout the carbon�carbon double bond. The lone pair on thenitrogen atom of theN,N-dimethylamino group can interact withthe π system and lead to partial double-bond character and

restricted rotation around the C�N bond. Other processes arealso possible, since the molecule is quasi-planar and the π systemcan extend throughout.61,62 For example, there will be restrictedrotation around the formal single bond that connects the estergroup to the formal carbon�carbon double bond. This will leadto the opposite configuration about the bond to the ester, butcalculations suggest that this is approximately 15 kJ mol�1 higherin energy, so it will not be observable. For the present purposes,there is the rotation about the carbon�carbon double bond,interconverting E and Z, then there are two rates of rotationaround the C�N bond available: one for the E form and one forthe Z. All these dynamic processes are readily studied by NMR.

The rotation around the C�N bond is an example of mutualexchange, in which the molecule is identical before and after theexchange and the equilibrium constant is exactly 1. However,rotation around the C�C partial double bond interconverts thedistinct E and Z forms of the molecule. The equilibrium constantwill be different from 1, and will depend on solvent. Therefore,MDACC is an excellent test sample with a number of measurablequantities to probe solvent effects.22,45,46,48,60

In this paper we present exchange rate measurements andelectronic structure calculations of the E and Z ground states andthe transition states for the three exchange processes. Exchangerates were measured in methanol, chloroform, acetonitrile, to-luene, dichloromethane, acetone, and tetrahydrofuran (THF). Awide range of rates was measured by using an extended tempera-ture range. The rates are measured both with line shape analysis inthe intermediate exchange regime and with selective-inversionmethods when the exchange rates are slow. The results show thatthe solvent dependence of the rate of the E�Z isomerization ismainly due to differences in the entropy of activation, and theseentropies of activation correlate with the polarity of the solvent.Furthermore, the E�Z equilibrium is also strongly affected byentropy. The molecule crystallizes as the E conformer, and theelectronic structure calculations predict that E is at a lower energythan Z in the gas phase. However, the dependence of theequilibrium constant on temperature shows that although the Econformer is the thermodynamic ground state, it is the Zconformer that has the lower enthalpy. This is just one exampleof how chemical exchangemeasurements onMDACCcan providea rich source of experimental data on solvent effects.

Figure 1. Structure of MDACC.

Figure 2. Comparison of the 13C solid-state NMR spectrum (top) ofMDACC in the E conformation and the liquid-state spectrum (bottom)of the equilibrium mixture of the E and Z conformations. Note thepresence of some spinning sidebands in the solids spectrum.

Scheme 1. Kinetic Scheme for Chemical Exchange inMDACC



’EXPERIMENTAL SECTION

MDACC (Figure 1) was synthesized by literaturemethods.58,59 Proton and 13C NMR spectra were run on aBruker DRX 500 with an inverse-geometry 5 mm probe.Temperatures were regulated with a BVT2000 controller andchecked with a thermocouple in an NMR tube. One-dimensionalspectra were obtained in solution, as well as a two-dimensionalHMBC spectrum to correlate the proton and carbon spectra. A13C cross-polarization/magic angle spinning (CP/MAS) NMRspectrum of the solid material was also obtained, using a BrukerAV 500 spectrometer equipped with a 4 mm MAS probe.52 Thepublished crystal structure confirmed that themolecule existed inthe E conformation in the solid state.52 Comparison of the 13CCP/MASNMR spectrum of the solid with the solution-state 13CNMR spectrum (Figure 2) allowed the carbon peaks of the E andZ forms in solution to be separately assigned. This assignmentagreed with the intensity differences in the solution spectrum,since the equilibrium constant for the E�Z exchange is not equalto 1.0. A two-dimensional HMBC experiment correlated thecarbon spectrum with the proton spectrum, allowing the latter tobe assigned unambiguously.

Exchange rate measurements in the slow exchange regimewere made with selective inversion experiments, in which onesite is inverted and its spin�lattice relaxation under bothexchange and T1 processes monitored. Data were analyzed usingthe CIFIT program which is available from ADB.63 EXSY spectrawere run to check the assignments, but all the quantitative datawere acquired using selective inversion. The proton spectra formthe intermediate exchange regime (Figure 3) could then besimulated as a function of temperature, using the MEXICO suiteof programs (also available from ADB).64 Excellent fits wereobtained, and there was no evidence of other dynamic processescontributing to themeasurements. It is difficult to define errors inthermodynamic parameters from exchange studies,13,63,65 but itis our opinion that barriers are typically reliable to (1 kJ mol�1

and entropies to (10 J K�1 mol�1.Electronic structure calculations mirror the observed crystal

structure. The calculations were done with both restrictedHartree�Fock methods and density functional (B3LYP) meth-ods at a number of levels: 6-311þG(d,p), 6-311þG(2d,p),

6-311þG(2df,2p), and 6-311þþG(2df,2p), using the Gaussian03 package66 on a dual-core Intel-based personal computerrunning at 3 GHz. The solvent effects were calculated usingthe polarized continuummodel (SCRF) method in Gaussian 03,for conditions appropriate for toluene, acetone, and ethanol. Thecalculated E geometry is very close to that observed in the crystal,and the bond lengths and angles (apart from the obviouschanges) are similar in the calculated Z conformation. Reportedenergies are directly from the program output, along with thezero-point and thermal corrections from the Gaussian log files.

’RATE MEASUREMENTS BY NMR

The measurement of rates as a function of temperature is theway we investigate the thermodynamics of the transition state.Absolute rate theory states that the rate of a reaction as a functionof temperature, T, is given by eq 1.

rate ¼ kBTh

e�ΔG‡=RT ¼ kBTh

eΔS‡=Re�ΔH‡=RT ð1Þ

In this equation, kB is Boltzmann’s constant, h is Planck’sconstant, and ΔG‡, ΔS‡, and ΔH‡ are the free energy, entropy,and enthalpy of activation. Therefore, an Eyring plot of thenatural logarithm of (rate/T) against (1/T) will give values ofΔH‡ from the slope and ΔS‡ from the intercept.

The rates of chemical exchange can be classified according tothe time scale of the NMR experiment. When a nucleus jumpsfrom one magnetic environment to another, its resonancefrequency is changed. This change in resonance frequency inthe NMR spectrum can then be compared to the exchange rate.Themost dramatic effects on the NMR spectrum occur when therate is comparable to the frequency difference. In this case, calledintermediate exchange, the spectral lines broaden, coalesce andthen sharpen into an average peak as the rate increases. Once pastcoalescence, the system is usually termed to be in fast exchange.Fast exchange is the most difficult regime from which to extractreliable data, and so it is not often studied for small molecules.However, the intermediate region is widely used. In this case, therate can be estimated by simulating the line shape as a function ofrate until a best fit is found to the experimental data.64,67�69

Slow exchange is the case in which the exchange rate is slowwith respect to the frequency difference, but it is comparable tothe relaxation rates. In this regime, rates are best measured byselective-inversion experiments: variations on the standard in-version�recovery T1 measurement.63,70�78 Two-dimensionalEXSY experiments can also be used.8,57,79 EXSY is an excellentqualitative experiment to map out exchange networks, but it isour opinion that the one-dimensional selective-inversion experi-ments give better quantitative data in a shorter time.

It is also important to use more than one method, to obtainrate data over as wide a range as possible,13,14,48 and to try tominimize the biases of a given experiment. The use of line shapemethods alone can be dangerous, since bad assumptions aboutthe natural line width will bias the rate one way in fast exchange,but the other way in slow exchange,80 leading to systematic errorsin the analysis of the Eyring plot.

For all the solvents, we did both selective inversion experi-ments at two or three low temperatures, and then a series of lineshape measurements at higher temperatures. The excellent datafrom the selective inversion experiments anchored the lowtemperature end of the Eyring plot, and the line shape dataextended it to higher temperatures.

Figure 3. van’t Hoff plot of the logarithm of the equilibrium constantbetween the E and Z conformations in deuterated THF against 1/temperature. Values of the differences in enthalpies and entropies for allsolvents are given in Table 1.



The thermodynamic data are summarized in Tables 1�4. Theerrors in the experimental numbers are difficult to estimate withany degree of certainty, since they are derived from multi-parameter least-squares fits.63,65 We estimate that the errors inthe enthalpy are(2 kJ mol�1, and the errors in the entropy are atleast (5 J mol�1 K�1. We disagree with a recent opinion81 thatimplies that entropies are equally well determined as enthalpies.As these authors point out, the error in the entropy from anEyring plot derives from two sources: the inherent error and theerror of the slope, which contributes due to the extrapolationback to 1/T = 0. Even for our best data,13 the relative error in the

entropy is large enough to preclude a detailed analysis. We arereluctant to put a great deal of confidence in entropies ofactivation.

’RESULTS AND DISCUSSION

The Equilibrium. Comparison of the solid-state NMR spec-trum of the E conformation and the solution spectrum of themixture (Figure 2) confirms that the E conformation is the moreabundant in all the solvents studied, so it must have the lower freeenergy. The electronic structure calculations also predict that Ewill have a slightly lower energy in the gas phase (Table 1), byapproximately 0.1 kJ mol�1, but not in the solvent models. Wedefine (Scheme 1) the equilibrium constant as the concentrationof Z divided by that of E, so its value is less than 1.0 in theseconditions. However, the van’t Hoff plot (Table 1 and Figure 4)shows the unusual situation of the equilibrium constant gettingfurther away from 1.0 as the temperature gets higher. Since theslope is proportional to the negative of the enthalpy of thereaction, this means that going fromE toZ is exothermic (ΔH0 < 0),even though the change in free energy (ΔG0) is positive. Insolution, the Z conformation has the lowest enthalpy, whereasthe gas-phase calculations suggest that E should be lower. Solventeffects and the associated entropy must be important. Table 1gives the measured values of the thermodynamic quantities for anumber of solvents, along with calculated values. Clearly thesolvent is playing a significant role in the equilibrium.The solvents are polar, and will interact with the dipole

moment of the molecule. The calculations predict (Table 6)that the Z conformation is more polar, and so it is expected to bemore stabilized by the solvents. A differential stabilization can beestimated from the measured ΔH0 and the calculated differencein the gas-phase energies. This is in the range of 2�3 kJ mol�1,but does not correlate directly with the dielectric constant of thesolvent. However, the solvents with the larger negative enthalpychanges also have the larger negative entropy changes. These twoeffects partly cancel each other, so that the equilibrium constants(governed by the free energy) are all comparable.The correlation of enthalpy and entropy changes makes sense.

The polar solvents help the transition from E to Z, but thistransition is accompanied by a significant decrease in the entropyof the system, so the overall change in free energy in this directionis positive. The solvent molecules stabilize the more polar Zconformation, but in doing so, lose entropy by becoming moreordered around the solute. It would be difficult to predict whicheffect will predominate, but in this case, the entropy wins.

Table 1. Thermodynamic Parameters for the Equilibriumbetween E and Z

solvent

dielectric

constant, ε

ΔH0

kJ mol�1

ΔS0 J z

(mol K)�1

ΔG0 kJ

mol�1

calculated

kJ mol�1

gas phase 0 0.1

toluene �0.6

CHCl3-d 4.81 �2.1 �10 1.0

THF-d8 7.5 �1.6 �8 1.0

CH2Cl2-d2 9.01 �1.6 �9 0.9

acetone-d6 20.7 �0.9 �6 0.8 �1.4

ethanol 24.3 �1.3

methanol-d4 32.7 �1.2 �5 0.5

acetonitrile-d3 37.5 �1.4 �7 0.8

Table 2. Thermodynamic Parameters for the E toZ Exchangearound the Double Bond

solvent

dielectric

constant, ε

ΔH‡

kJ mol�1

ΔS‡ J

(mol K)�1

ΔG‡ at 300

K kJ mol�1

calculated

kJ mol�1

gas phase 0 88.0

toluene-d8 2.41 46 �72 67 72.7

CHCl3-d 4.81 49 �44 63

THF-d8 7.5 36 �80 60

CH2Cl2-d2 9.01 46 �49 61

acetone-d6 20.7 50 �30 60 57.8

ethanol 24.3 57.4

methanol-d4 32.7 45 �24 52

acetonitrile-d3 37.5 49 �21 55

Table 3. Thermodynamic Parameters for the Exchange of theN,N-Dimethylamino Group in the E Conformation

solvent

dielectric

constant, ε

ΔH‡

kJ mol�1

ΔS‡ J

(mol K)�1

ΔG‡ at 300

K kJ mol�1

calculated

kJ mol�1

gas phase 0 46

toluene-d8 2.41 49 3 48 50.7

CHCl3-d 4.81 55 14 50

THF-d8 7.5 53 16 48

CH2Cl2-d2 9.01 55 23 49

acetone-d6 20.7 56 25 48 55.3

ethanol 24.3 55.5

methanol-d4 32.7 56 36 46

acetonitrile-

d3

37.5 58 41 47

Table 4. Thermodynamic Parameters for the Exchange of theN,N-Dimethylamino Group in the Z Conformation

solvent

dielectric

constant, ε

ΔH‡

kJ mol�1

ΔS‡ J

(mol K)�1

ΔG‡ at

300 K kJ mol�1

calculated

kJ mol�1

gas phase 0 55.0

toluene-d8 2.41 63 32 53 63.7

CHCl3-d 4.81 68 43 55

THF-d8 7.5 69 51 53

CH2Cl2-d2 9.01 73 64 54

acetone-d6 20.7 73 68 53 72.4

methanol-d4 32.7 77 88 52

acetonitrile-

d3

37.5 78 92 51



The Exchange. There are two types of exchange processesactive in this molecule: the rotation around the nominal carbon�carbon double bond (interconverting the E andZ conformations),and rotation of the N,N-dimethylamino group around the car-bon�nitrogen bond. The latter occurs under slightly differentcircumstances in the E and Z conformations. Tables 2, 3 and 4present the experimental data and Figure 5 shows one of theEyring plots.The transition states have the expected geometries (Figure 5,

and Supporting Information). In the transition state for the E�Zisomerization, the N(4)C(3)C(7) plane is twisted by 89 degreesfrom the C(1)C(2)C(10) plane, and the C(2)�C(3) distancegrows to 1.467 Å from 1.401 or 1.400 in the ground states.Because the double bond has been broken, the C(2)�C(1),C(2)�C(10), and the C(3)N(4) bonds all shorten. The sub-stituents around the nitrogen of the N,N-dimethylamino groupremain in a plane, suggesting some conjugation between the lonepair on nitrogen and the p orbital onC(3). The transition states forthe E�E0 and Z�Z0 processes both have a plane of symmetry:C(7), C(3), C(2), C(1) and C(10) all lie in a plane, and it is onlythe N,N-dimethylamino group that is twisted. Note that althoughMDACC has a twisted ground state, this is probably a steric effect.Calculations suggest that the simple amino analogue (with themethyl groups replaced by hydrogens) is planar. There is alsosignificant pyramidalization at the nitrogen, which is different atthe E�E0 and Z�Z0 transition states. The C(3)�N(4) bond islonger than in the ground state in both cases, but the C(2)�C(3)bond (the nominal double bond) is shorter, as would be expected.The E to Z exchange has the highest barrier (Table 2), and the

clearest trend: the dimethylamino rotations show a smaller solventdependence. The enthalpy of activation is roughly constant for allsolvents, ranging from 45 to 50 kJ mol�1. This is significantlydifferent from the difference in energy in the calculated gas phase(Table 1) values. However, the entropies of activation aresubstantial (from �21(mol K)�1 in acetonitrile to �68 J (molK)�1 in toluene) and negative. This means that the transition stateis more ordered than the ground states, but the differentialordering decreases with increasing polarity of the solvent. Thetransition state is usually regarded as more zwitterionic than theground state, which is supported by the calculated dipole mo-ments. The values (Table 6) for E and Z are 5.12 and 6.19 D,respectively, and the value for the transition state is 8.71 D.

The measured barriers to rotation around the C�N bondjoining the N,N-dimethylamino group to the ethylene aresignificantly different in the E and Z conformations (Tables 3and 4). The values for both the enthalpy and the free energy the Econformation are lower than for the Z conformation. In bothcases, the enthalpy rises with the solvent polarity. The calcula-tions also predict that the barrier in the Z conformation will behigher, but the calculated numbers are lower than the measuredenthalpies. The measured entropies of activation are positive forthese processes, consistent with the prediction that these transi-tion states have lower dipole moments than the ground states.The transition state is more disordered than the ground states,but as the solvent polarity increases, the differential disorderingincreases. This trend with solvent polarity is the same as thatobserved for the E to Z exchange.The Calculations. The data in the tables suggest that the

electronic structure of the transition states is complicated.18,50,82

There is a significant change in the calculated transition stateenergies on going from the 6-311þG(d,p) basis set to higherbasis sets, but further calculations with better models indicatethat the 6-311þþG(2df,2p) basis is sufficient (see Table S1,Supporting Information). There are also significant thermal andzero-point corrections (Table 5), but they are similar for allprocesses, especially the two dimethylamino rotations, so they donot change the conclusions. The calculated values for the E to Zprocess (rotation about the partial double bond) are significantlyhigher than the experimental values of eitherΔH orΔG, for boththe gas phase and the toluene model (Table 2). The models of

Figure 5. Geometries of calculated ground and transition states. Toprow: the E and Z ground states; middle: the E to Z transition state;bottom: the transition states for the rotation of the N,N-dimethylaminogroups in the E and Z conformations.

Figure 4. Eyring plot for the E to Z exchange for a number of solvents.



polar solvents improve the situation, but the discrepancies arestill substantial. The calculated dipole moment for this transitionstate is very large (8.7 D, Table 6), so even at a fairly high level oftheory, the calculations are not very reliable. For the rotationabout the C�N bond of the N,N-dimethylamino group, thetransition state is less polar than the ground state, since the fullydelocalized π bonded structure is disturbed and the push�pulleffect attenuated. In this case, the calculations predict that theprocess will be more hindered in the Z conformation than in theE, as was observed experimentally and may be seen in Figure 5.The gas-phase calculations underestimate the energy of thetransition state, but the solvent models are quite successful(Tables 2�4).

’CONCLUSIONS

There are three distinct exchange processes in MDACC. Oneis the rotation about the central carbon�carbon double bond,which interconverts the E and Z conformations. The N,N-dimethylamino group in each of the two conformations alsoexperiences hindered rotation about the bond that joins thenitrogen to the ethylene nucleus. These three processes give usan exquisitely sensitive probe of the effects of solvent on the ratesof the three exchange processes. The experimental values arebolstered by electronic structure calculations, which providecomplementary information about the processes. The solventdependence is principally due to entropy effects, which arerelated to the solvent polarity and the dipole moments of theground and transition states. Solvent model electronic struc-ture calculations provide a good picture of the exchange involvingthe N,N-dimethylamino group, but the transition state connect-ing the E and Z conformations is not well done, perhaps becausethe calculations suggest that this is a very polar state and willperturb the solvent significantly. Finally, this system shows theunusual situation where the more stable conformation does nothave the lowest enthalpy. Going from the Z to the E conforma-tion decreases the Gibbs free energy, but increases the enthalpy.

’ASSOCIATED CONTENT

bS Supporting Information. Geometries (in xyz file format)of the stationary points on the potential energy surface (twoground states and three transition states) obtained from high-level calculations. There is also a table comparing the calculationsat a number of different basis sets, using B3LYP approaches. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail address: [email protected].

Present Addresses†School of Pharmacy, Pharmaceutical Sciences Dept., KittrellHall, Hampton University, Hampton, VA 23668, United States.‡Dept. of Chemistry, McGill University, Montreal, Quebec,Canada H3A 2K6.

’ACKNOWLEDGMENT

Wewould like to thank Brian G. Sayer and DonaldW. Hughesfor technical assistance, and Dr. Tammy Dwyer for the gift of aninitial sample. We are grateful to the Natural Sciences andEngineering Research Council of Canada (NSERC) for financialsupport.

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Table 5. Contributions to the Calculated Energies in the GasPhase at the B3LYP/6-311þþG(2dp,2p) Level of Theorya

conformation energy zero point thermal

Z 0.14 0.0 0.0

transition state E�Z 88 �3.6 �4.2

transition state of dimethylamino

rotation (E conformation)

46 �4.4 �5.8

transition state of dimethylamino

rotation (E conformation)

56 �4.8 �6.0

aThe values are relative to theE conformation and are all in kJ mol�1.

Table 6. Calculated Dipole Moments (in Debye) of theConformations

conformation dipole x dipole y dipole z total

E �4.08 �2.99 0.76 5.12

Z 4.91 �3.93 0.01 6.29

EZ 7.23 �4.86 �0.16 8.71

EE0 �1.92 �3.27 0.0 3.79

ZZ0 �1.54 �2.77 0.0 3.17



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Solvent Effects on Chemical Exchange in a Push–Pull Ethylene as Studied by NMR and Electronic Structure Calculations