Journal of Molecular Structure, (Theochem), 153 (1987) 67-74 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

AN AB INITIO STUDY OF THE POSSIBLE ISOMERS AND CONFORMATIONS OF N-(TRIFLUOROMETHYL)FLUORO METHANIMINE, CF,N=CHF

MATTHEW CLARK and JOSEPH S. THRASHER

Department of Chemistry, The University of Alabama, Tuscaloosa, AL 35487 (U.S.A.)

(Received 23 November 1986)

ABSTRACT

The relative energies of the four possible isomers and conformations of N-(trifluoro- methyl)fluoromethanimine, CF,N=CHF, have been studied by ab initio calculations at the STO-3G, 3-21G, 4-31G and 4-3lG* levels. While the STO-3G level fails to reproduce the experimentally determined order of conformational stability, the split-valence basis sets correctly predict the E-eclipsed conformation to be the most stable. Transition state energies have also been calculated, and the barriers to isomerization are in agree- ment with experimental data.

INTRODUCTION

Methanimine derivatives with electronegative substituents such as fluor- ine, chlorine or CF3 groups are exceedingly stable when compared to the parent compound, HN=CH2 [l]. Recently, Lentz and Oberhammer repor- ted a number of new N-(trifluoromethyl)methanimines from the e-addition reactions of trifluoromethyl isocyanide with hydrogen halides [2] . These reactions were made possible only by the vastly improved synthetic route to CF3NC [3]. Lentz and Oberhammer were able to show by “F-NMR that both the E and 2 isomers of the CF,N=CHX derivatives were present. In the case of CF,N=CHF, the 4JFF coupling constants (5.0 Hz for the major isomers vs. 10.5 Hz for the minor isomer) indicated that the pre- dominant isomer was most likely the E isomer, but this had to be con- clusively shown by electron diffraction. The experimentally determined conformations of the major and minor isomers are labeled in Fig. 1. The free enthalpies of activation of isomerization were determined by variable- temperature NMR studies; the values for CF,N=CHF are given in Table 1 121.

A number of ab initio studies on methanimine and its seven different fluoro derivatives [4-71, i.e., mono-, di- and perfluoro, have appeared in the literature. These studies, as well as experimental data, have indicated that isomerization takes place through inversion about the nitrogen. The barrier to rotation is much higher than that of inversion due to the neces-

0166-1280/87/$03.50 o 1987 Elsevier Science Publishers B.V.

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F’ F’

F-C / /

F’ \ /H FFTc\N_C/F

N-C

‘F ‘H

E-eclipsed Z-eclipsed (Major)

F

IbF LF

F I.

‘\ /H F’+ /F N-C N-C

‘F ‘H

E-staggered Z-staggered (Mmar)

Fig. 1. Possible isomers and conformations of CF,N=CHF.

TABLE 1

Thermodynamic data for CF,N=CHFa

Compound Tco AG2c.s t

AGTCO AGTCO EA(=~C) AE( talc) (K) (KJ mol-‘) (kJ mol-‘) (kJ mall’) (kJ mall’) (kJ mall’)

E-CF,N=CHF 393 6.8 + 0.5

81.3 + 1.3 Z-CF,N=CHF 76.8 f 1.3

5.5 f 0.5 94.0 86.0

8.0

aKey: T,o, coalescence temperature; AGZgs , difference between the free enthalpies of formation of the E and 2 isomers at 298 K; aGLco , free enthalpies of activation of the isomerization between E and 2 isomers at Tco; AG,oo, difference between the free enthalpies of formation of the E and 2 isomers at Tco ; EA, obtained from transition state optimization at 4-31G*//4-31G level; AE, difference between EA for E and 2 isomers.

sity of breaking the carbon-nitrogen double bond and the concomitant electronic changes [6, 8, 91. Because of these studies, we have limited our study of transition states among the isomers of CF,N=CHF to the inversion mechanism, which reproduces the experimental result well [ 21.

COMPUTATIONAL DETAILS

The four geometries of CF,N=CHF were fully optimized in C, symmetry with the ab initio program GAUSSIAN 80 [lo] using STO-3G, 3-21G and 4-31G basis sets. For the calculations at the STO-3G level the CF3 fluorines were assumed to be equivalent, while at the 3-21G and 4-31G levels two of the CF3 fluorines were made equivalent and one unique. Calculations allowing free rotation of the CF3 group showed no tendency to deviate

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from C, symmetry. Geometries were optimized until the variations in bond lengths and angles met the default fourfold criteria of the “Berny” optimiza- tion [ll] . These default optimization thresholds are: maximum force, 0.00045 hartree Bohr-‘; r.m.s. force, 0.00030 hartree Bohr-‘; maximum displacement, 0.00180 Bohr; r.m.s. displacement, 0.00120 Bohr. Transition states were optimized with the CF3 group both eclipsed and staggered, retaining C, symmetry. Again, test calculations indicated no tendency to leave C, symmetry. The search for each transition state was carried out at the three basis levels by adjusting both the C-N and N=C bond lengths and the N-C-N angle until the forces and eigenvalues reached the default criteria of the “Bemy” transition state calculation. The remaining geometric parameters were taken from the optimization at that respective level. Single point calculations were then carried out at the 4-31G* level on all 4-31G geometries.

RESULTS AND DISCUSSION

The geometric parameters for CF,N=CHF optimized at the STO-3G, 3-21G and 4-31G levels are given in Tables 2 and 3; Figs. 2 and 3 show the calculated structures of the conformers and isomers as well as the calculated transition state geometries. The total energies are given in Table 4. The calculated geometric parameters are in good agreement with the electron diffraction data, which are also given in Tables 2 and 3. Notable deviations occur for the N=C double-bond length and the CNC angle when using the split-valence basis sets, the length being 0.04 A too short in the optimized structures and the angle 5” larger than the observed value. It is interesting to note that geometry optimization at the STO-3G level leads to closer values for these two parameters, but the C-N and C-F bond lengths are too long by 0.09 and 0.03 A, respectively.

While the energies of the optimized STO-3G structures incorrectly predict the Z-staggered form to be the most stable, the split-valence optimizations correctly reproduce the experimental finding that the E-eclipsed form is most stable. The ordering of the second most stable isomer is sensitive to changes in the basis set. Split-valence calculations at the 3-21G and 4-31G levels indicate that the Z-staggered form is more stable than the E-staggered form by 0.00327 and 0.00199 hartree. Upon adding polarization functions for single point calculations at the 4-31G* level the order reverses itself; however, the energy difference between the isomers is only 0.0002 hartree.

The eclipsed conformation of methyl or trifluoromethyl groups adjacent to double bonds is not unusual; the structure of the parent compound CH3N=CH2 as determined by microwave spectroscopy also shows a prefer- ence for the eclipsed conformation [ 121. Similarly, microwave and electron diffraction data indicate that the compound CFJNO [13, 141 takes the eclipsed conformation, as do CH$(O)F, CH&(O)CI, CH&(O)CN [15] and CH3CH==CH2 [16]. Pauling has rationalized the preference for the

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TABLE 2

Geometric parameters of E-eclipsed CF,N=CHF in a and degrees at various basis levela

Experimental [ 2 ] SCF/STO-3G SCF/3-21G SCF/4-31G

C-F(CF,) l-332(4) 1.369 1.337 C-F’( CF,)

b b 1.360

N-C 1.414(7) 1.505 1.398

:z(CF) 1.312(10) 1.277(7) 1.273 1.344 1.234 1.333 C-H 1 .oga 1.10 1.069 ,!J?‘CN b b 112.4 LFCF 107.8(4) 108.3 108.2 LCNC 117.3 115.6 123.1 ,!NCF 117.7(14) 119.4 120.7 LNCH 125a 126.7 127.3

aParameter not optimized. bAll CF, fluorines assumed equivalent.

1.346 1.372 1.392 1.236 1.338 1.069

113.3 108.2 124.0 119.7 128.4

TABLE 3

Geometric parameters of E and 2 isomers of CF,N=CHF in A and degrees

C-F( CF, ) C-F’(CF,) N-C N=C C-F( CF) C-H ,!_F’CN LFCF .&NC ,!.NCF .!_NCH

E eclipsed E eclipsed Eetaggered Z-eclipsed Zstaggered expt. [2] SCF/4-31G SCF/4-31G SCF/4-31G SCF/4-31G

1.332(4) 1.346 1.362 1.349 1.358 b 1.372 1.338 1.361 1.343

1.414(7) 1.392 1.397 1.393 1.400 1.277(7) 1.236 1.237 1.234 1.235 1.312(10) 1.338 1.338 1.354 1.355 l.Oga 1.069 1.071 1.065 1.065

b 113.3 111.0 115.1 110.1 107.8(4) 108.2 106.1 108.0 107.2 117.3(9) 124.0 122.8 131.3 126.9 117.7(14) 119.7 120.7 125.1 123.9 125a 128.4 128.7 123.3 124.0

aParameters not optimized. bAll CF, fluorines assumed equivalent.

eclipsed conformation on the basis of the bent bond model, indicating that this form reduces interaction with the xelectrons of the double bond

v71. The geometries of the four conformers exhibit only minor differences in

bond lengths and angles. In both eclipsed conformers, the CF3 fluorine opposite the nitrogen lone pair exhibits a longer C-F bond than the other two; the staggered conformers show the two fluorines opposite the nitrogen lone pair to have longer C-F bonds than the other CF3 fluorine. In both conformations of the E isomer the difference between the CF bond lengths

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N-C

Ff-C ’ ‘F ‘F,

Z-eclipsed

‘; H

C-N-C’ F;

\F

eclipsed TS

\ (86.0 kJ/mol)

F,C \ H

N-C’

N-C’

H

F’ / \

‘F

‘C F IF

E-staggered Z-staggered

Fig. 2. Eclipsed transition state with energy barriers.

‘F E-eclipsed

/H N-C

FF-C ’ ‘F

/

‘F’

(82.2 kJ/mol) Z-eclipsed

-N-C

‘F

f stoggered TS

/ (86.0 kJ/mol)

F,C \ H

N-C’

\F

/H N-C

‘F

‘;’

E-staggered Z-stoggered

Fig. 3. Staggered transition state with energy barriers.

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TABLE 4

Total energies of CF,N=CHF (hartree)

SCF/STOSG SCF/3-21G SCF/431G SCF/4-31G*’

E-eclipsed -521.26355 -525.64169 -527.80317 -528.01674 E-staggered -521.26365 -525.63753 -527.80054 -528.01396 Z-eclipsed -521.26247 -525.64115 -527.80192 -528.01254 Zstaggered -521.26500 -525.64080 -527.80252 -528.01368 Eclipsed transition -521.21847 -525.62081 -527.78095 -627.98094

stateb Staggered transition -521.21899 -525.62041 -527.78045

stateb -527.98122

aSingle point at 431G optimized geometries. bWith respect to hydrogen.

in the CF3 group is approximately 0.025 A, while the difference averages only 0.014 A in both conformations of the 2 isomer. In addition to the bond lengthening, the fluorine or fluorines opposite the nitrogen lone pair exhibit a slightly more negative charge by Mulliken analysis than the other fluorine(s) of the CF3 group as shown in Table 5. The effect is most pro- nounced in the E-eclipsed conformation where the charge difference between the fluorines is 0.031 electrons; however, accumulation of charge on this fluorine is hampered by the highly positive adjacent carbon atom.

The bond elongation and increased negative charge of the fluorines opposite the nitrogen lone pair are consistent with a small amount of elec- tron donation from the nitrogen lone pair into the carbon-fluorine anti- bonding orbital, i.e., a small contribution from an anionic hyperconjugation resonance structure. This effect, also known as the anomeric effect, has previously been studied and observed in ab initio calculations on other neutral molecules [ 18-201. The E-eclipsed form allows maximum overlap between the nitrogen and carbon p orbitals and thus the stabilizing effect is most pronounced in this conformer. The Z-eclipsed form suffers from the steric interaction between the eclipsed fluorines which increases the core- core repulsions by 2.0 kcal mol-’ due to the calculated 2.66 A distance between the fluorines. Oberhammer and Lentz assumed the staggered con- formation of the Z isomer in fitting to the electron diffraction data of CF,N=CHF as well as CF3N=CF2 [2].

Since the two observed isomers may be viewed as differing by an inversion about the nitrogen without rotation of the CF3 group, two transition states were optimized, one transition state with the CF3 fluorines staggering the hydrogen, the other with the CF3 group eclipsing this hydrogen. The results of the transition state searches are depicted in Figs. 2 and 3. As expected both the C-N bond and the C=N double bond shorten in the transition state (4-31G, eclipsed transition state with respect to hydrogen d(CN)

13

TABLE 5

Atomic charges and dipole momen@

Atom E-eclipsed E-staggered Z-eclipsed Z-staggered Eclipsed Staggered transition transition stateb stateb

N -0.589 -0.586 -0.613 -0.608 -0.681 -0.682 C(N=C) 0.525 0.537 0.510 0.514 0.548 0.542 H 0.234 0.229 0.246 0.250 0.219 0.224 F(CF) -Q.353 -0.354 -0.361 -0.363 -0.368 -0.362 C(CF,) 1.363 1.362 1.396 1.388 1.469 1.466 F(CF,) -0.383 -0.402 -0.390 -0.395 -0.395 -0.400 F’(CF,)C -0.414 -0.383 -0.396 -0.390 -0.397 -0.387 Dipoled 2.536 2.519 3.010 3.043 2.946 2.798

‘Mulliken analysis at 4-31G*//4-31G. bWith respect to hydrogen. ‘Unique fluorine of CF, group. dDebye.

= 1.352 A, d(C=N) = 1.208 A, LCNC = 175”; staggered transition state d(CN) = 1.350 A, d(C=N) = 1.210 A, LCNC = 179.6”).

Transition state searches at the split-valence levels were carried out by optimizing the N-C bond length, the N=C double-bond length and the C-N-C angle. The remaining bond lengths and angles for the transition state between the E-eclipsed and Z-staggered forms were taken from the Z-staggered optimization at the appropriate basis level and for the E-stag- gered and Z-eclipsed forms the values were taken from the E-staggered opti- mization. The rationale is that the transition state geometry is closer to the higher energy isomer. Transition state calculations using values from the other isomer in each case showed minimal energy differences.

The calculated energy barriers to the transition state at the 4-31G* level, 94.0 and 86.0 kJ mol-’ for the E and 2 isomers, respectively, agree well with the values measured by NMR experiments (81.3 and 76.8 kJ mol-‘) as do the differences between the values in each case (8.0 kJ mol-’ cal- culated vs. 6.8 kJ mol-’ experimental at 298 K) [2]. The thermodynamic results are summarized in Table 1. The overestimation of the transition state barriers is expected due to the absence of corrections for zero point vibrational energy. The total energies summarized in Table 4 show that at the 4-31G*//4-31G level the staggered transition state is marginally favored over the eclipsed transition state, although the observed species would iso- merize via the eclipsed transition state if there were no CF3 rotation. Since the barrier to CF3 rotation is considerably lower than either barrier to inversion, it is possible for an inversion to be preceded by rotation. In both transition state cases, it is predicted that the barrier for going to the tran- sition state from the E form is greater than the barrier from the 2 form, in agreement with the 15:l ratio of isomers (E:Z) observed in NMR spectra PI-

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CONCLUSIONS

Ab initio calculations at the 4-31G* level correctly reproduce the experi- mental conformational stabilities of the isomers of CF,N=CHF, as well as the energy barrier for isomerization. Calculations at lower split-valence levels reproduce the conformational stabilities, but understimate the barrier to isomerization. All calculations, except at the STO-3G level, underestimate the C-N bond distance by about 0.04 A. The most stable form is found to be the E-eclipsed conformer, in agreement with the electron diffraction data; this finding is consistent with the structures of many analogous com- pounds.

ACKNOWLEDGMENTS

A generous grant of computer time from the UA Seebeck Computer Center is gratefully acknowledged. M. C. acknowledges the UA Graduate School for a Graduate Council Research Fellowship.

We acknowledge Tripos Associates, Inc., St. Louis, Missouri for a grant of SYBYL Molecular Modeling System, as well as the donors of the Petroleum Research Fund, administered by the American Chemical Society.

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