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A comparative theoretical study of trifluoromethyl and methyl cyanides and isocyanides and the cyanide-isocyanide isomerization process
Volume 55, numbefl
A COWARATIVE AND ISOCYANIDES
CHEMICAL
THEORETICAL
PHYSICS
1 April 1978
LETTERS
STUDY OF TRIFLUOROMETHYL
AND THE CYANIDE-ISOCYANIDE
AND METHYL CYANIDES
ISOMERIZATION
PROCESS
J.B. MOFFAT Departntettt of Citetttistry and Gttelph-Waterloo Waterloo. Ontario. Canada NZI. 3GI Received
15 November
Certtre for Graduarc
Work in Chemistry.
iJttinmit_v
of Waterloo.
1977
Ab initio (STO-3G) geometry-optimized calctdations have been performed on methyl and trifluoromethyl cyanide and isocyanide, and energies of isomerization of 24.1 and 11.5 kcal/mole, respectively have been calculated. Activation barriers of 87.8 and 80.0 kwl/mole are predicted for the isomerization of methyl and tritluoromethyl cyanide, respectively, by esamining various features of the potential surface. The geometries, atomic charges, overlap populations, and molecular orbitals are employedto compare the two cyanides, isocyanides and transition species.
1. Introduction The structural isomerizations of molecules are important processes for both experimental and theoretical study. Because of the relative simplicity of the reacting system, extensive experimental data is obtainable, at least in principle, and a variety of theoretical techniques can be applied to provide desirable ancillary information. It is useful, as well as interesting, to examine systems where little or no experimental or theoretical information is available_ Here theoretical studies can hopefully assume-a predictive role for the experimentalist. The cyanide-isocyanide isomerization is an example of a unimolecular process where sufficient experimental and theoretical work has been performed on several cyanide-isocyanide systems, for example, methyl cyanide-isocyanide, to provide some insight into relative stabilities, possible isomerization mechanisms, rates, activation energies and related quantities. One of the systems for which somewhat less information is available is that of vinyl cyanide-isocyanide. Recent calculations have predicted differences in energy of 2 1.4 and 17.7 kcal/mole with an STO-3G and a 6-3 iG basis set, respectively, and energy barriers from the cyanide and isocyanide of 74.5 and 53.0 kcal/ mole, respectively ]I].
Recently, Lee and Willoughby [2] have reported the results of *.arious spectroscopic studies of trifluoromethyl isocyanide. This compound was first prepared ten years ago 13,4], and has been fo&d to be easily polymerized and isomerized to the cyanide. The vibrational spectra of related compounds trifluoromethyl cyanide [5,6], methyl isocyanide [7,8] and methyl cyanide [S] have been reported previously. Since no quantitative experimental information is available on the relative energies or rates of isomerization in the case of tritluoromethyl cyanide-isocyanide, but both isomers have been prepared, this appears to be a suitable system for predictive theoretical examination. In addition, the availability of experimental data for methyl cyanide and isocyanide provides opportunity for interesting comparisons. Ab initio SCF calculations [9] employing the STO3C basis and standard exponents were employed throughout. Geometry optimization of all bond lengt!rs and angles to 0.001 A and O-lo, respectively, was performed on both the cyanide and isocyanide. The CCN and CNC bonds were both held at 180”. An approximate energy surface for isomerization was constructed by fixed the CN bond on the x axis with the centre of the bond at the origin and allowing the trifluoromethyl group to move away from the CN bond along various paths perpendicular to that bond. Additional 125
Volume 55, nuniber 1
CHEMICAL PHYSICS LETTERS
1 April 1978
points on the potential surface were obtained by fir-
tances in the cyanides differ by almost 0.05 A, while
ing the trifluoromethyl group at a particular distance from the origin with the line joining the origin to the -CF, group passing through the N atom. This line is then rotated about the origin until it passes ‘through the carbon atom 180° later. Energies are calculated at selected intervals. For comparison purposes, similar calculations were performed on methyl cyanide and isocyanide.
the C-N lengths in the isocyanides differ by 0.018 A.
2. Results and
Somewhat surprisingly the bond angles in the two cyanides are quite similar, as are those in the two isocyanides.
The cyanides are predicted to be, as expected, more stable than the isocyanides (table 1). However a remarkable difference in the energies of isomerization for the two cyanides is observed, that for methyl cyanide (24.1 kcallmole) being more than twice as large as that for trifluoromethyl cyanide (11.5 kcal/mole). No experimental data are available for the energy of isomerization of trifhtoromethyl cyanide. Benson [ 1 l] estimated an energy difference of 15 kcal/mole between methyl cyanide and isocyanide. Liskow et al. 1121 found a value of 17.4 k&/mole from ab initio calculations. Very recently, Pritchard and co-workers 1131 have reported an enthalpy of isomerization of 23.7 + 0.14 kcal/mole for the isomerization of methyl cyanide to isocyanide. The present calculated valne of 24.1 kcal/mole is in surprisingly good agreement with
discussion
2. I. csliculated geometries isocyanides
of the cyanides
of isomerikation
2.2. Energies
and
The optimized nuclear configurations for trifluoromethyl cyanide and isocyanide are summarized in table I together with those for methyl cyanide and isocyanide for comparison. No experimental structural data are available for the former two molecules. The cyan0 bond lengths and isocyano bond lengths are slightly smaller in the methyl molecules compared to those of trifluoromethyl. However, the C-C dis-
Table 1 The optimized nuclear configurations and total electronic energies (hrcludmg nuclear repulsion) of trifluoromethyl cyanide and isocyanide F3CCN bond lengths (A) (iso) cyan0 CN cc
1.534
CN
-
xc a)
1.371
bond angles (deg) LXCC a) LXCN a)
~xcx a) electronic energy (hartree) AE
126
1.156
110.2
108.7
i13CCN
1.154 (1.157) b) 1.486 (1.458) 1.088 (1.103)
F3CNC
1.178 1.464 1.367
H3CNC
1.171 (1.166) 1.446 (1.424) 1.092 (1.101)
110.1 (109.5)
108.9
-422.63651
-130.27155
11.5
24.1
b) Values in parenthesesare experimental data from ref. [lo].
109.7 109.2 -422.61817
109.9 (109.1) 109.1 -130.23319
1 April 1978
CHEHICAL PHYSICSLETTERS
Volume 55, number 1
the experimental value of Pritchard. As Liskow et al.
[Me+“‘341
[ 121 point out, the correlation energies of MeCN and
for the isocyanides. Previous calculations [l] on vinyl cyanide and isocyanide predicted ionic characters represented by [C,Hp258] [CN-0*‘58] and [C,H3i0*4gg [NC-“-4gg], respectively. The ionic character& each of the three isocyanides is approximately double that found for the corresponding cyanide. However, the overlap populations for the nitrile bond are almost identical in the methyl and TFM molecules. In contrast, the o overlap population for the isocyanide bond is considerably larger in methyl isocyanide, while the ‘IToverlap populations are similar in both isocyanides. This suggests that the fluorine atom is capable of perturbing the u density without altering that of the z electrons.
MeNC are presumably approximately equal, and the differences of the calculated electronic energies from the exact single-determinant (Hartree-Fock) values may also be nearly the same [ 141, so that the present value for the energy difference is expected to be relatively independent of the basis set and hence reasonably reliable. In this regard it may be noted that the energy difference for vinyl cyanide-isocyanide was previously calculated ]l] as 21.4 and 17.7 kcal/mole with an STO-3G and a 6-3 1G basis set, respectively.
2-3. Charges Qtzdpopul~n~ons
[NC-0.2341
and [TFM+0.204 ] [NC-0.20?]
The net atomic charges and overlap populations are summarized in tables 2 and 3. The nitrile nitrogen
2.4. Molecular orbitals of the c_vanidesand isocyanides
atom in methyl cyanide is considerably more negative
As expected, the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of both methyl and trifluoromethyl cyanide are of approximately 1~symmetry (fig. 1). In the cyanides the orbitals immediately below the HOMO are of o symmetry_ In the isocyanides the LUMO are
than that in trifluoromethyl cyanide while the isonitrile carbon in the former molecule is much less positive than that in the latter. The ionic character can be represented as [Me+“-i23] [CN-u-1*3] and [TFM+0-0g7] [CN- uo97] for the cyanides and Table 2 Net atomic charges
methyl TFM C) a)X=ForH.
Transition state
isocyanide
Cyanide x a)
Cd)
C
N b)
x a)
c d)
N
c b)
x a)
cd)
N
C
+0.101 -0.122
-0.181 +0.464
+0.075 i0.041
-0.198 -0.138
+0.099 -0.122
-0.062 +0.568
-0.332 -0.37 1
+0.098 +0.167
+0.112 -0.099
-0.135 +0.504
-0.196 -0.196
-0.012 -0.010
b) Terminal atom.
C) TFM = trithroromethyl.
d) C attached to X.
Table 3 Overlap populations Cyanide
methyl TFM b)
a) Terminal atom. d, Sum of *p,-2px
Isocyanide
Transition state
cc
CN a)
CN
NC aI
c ck
C C)N
CN
0.170 0.0090 0.154 0.0108
0.098 0.444
0.157 0.0054
0.226 0.388
-0.010 +0.107
0.053
==0
0.440
0.139
0.372
+0.092
-0.015 +0.053 -0.01 ltO.O4?
0.083 0.333 0.092 0.333
0.095
0.0195
b) TFM = tritiuoromethyl. C) Carbon of methyl or tritluoromethyl group. and *py-*p,, overlap populations.
127
Volume 55, number 1
CHEMICAL
PHYSICS
3
+Q50 +Q40 +a30
+ozo
: F F;C-GIN F
-038
-
F F\-C-N F’
%C
1
Fig. 1. LUJfO and HOMO energies for cyanides, and transition state species.
1 April 1978
2.5. The transitiDrtstate The results of calculations in which the methyl or trifluoromethyl group orbit about the centre of the CN bond are shown in figs. 2 and 3. The abscissa represents the angle (8) made by the CN bond and the line (R) joining its centre and the carbon atom of either the methyl or trifluoromethyl group. Values of 8 of 0” and 180” represent the isocyanide and cyanide, respectively. Each line in f&s_ 2 and 3 traces the energy changes, with respect to the cyanide, and for a given fied length of the line R as the isornerization from isocyanide to cyanide, or vice versa, occurs. The similarity of the two graphs is readily evident. Two minima as well as the maximum, the latter representing the activation barrier, can be clearly seen. The minimum barrier for the MeCN-MeNC isomerization occurs at an R value of 1.70 A, a 6 value of loo’, and an energy change of 87.8 kcal/mole. For the F,CCNF,CNC isomerization, the minimum barrier of 80 kcal/mole occurs for an R value of 1.76 a at 100”.
isocyanides,
again of rr symmetry but now the HOMO are of o symmetry and the orbitals immediately below the HOMO are of o symmetry_ As fig. 1 illustrates, isomerization of either cyanide to the corresponding isocyanide involves a small increase of the LUMO (T*) energy and an even smaller decrease in the energy of the occupied rr orbital. However the energy of the occupied u orbital increases substantially and crosses that of the occupied 7~orbiial so that the occupied u orbital becomes the HOMO in the isocyanides. It should also be noted that substitution of fluorine for hydrogen in the isocyanide produces a stabilization of the highest occupied o orbital, but an even greater decrease in energy of the highest occupied of orbital. Most noticeable however is the marked x-stabilization on substitution of fluorine for hydrogen in the cyanide, while the u orbital remains almost unchanged in energy- This appears to be the reverse of the so-called perfluoro effect which has been observed in planar ethylenic molecules [ 151.
128
LETTERS
0
I
zo
I 40
I 80
I
80 8
I
loo (deg.1
I
I
I20
I40
Ia3
I
Fig. 2. Energy relative to that of methyl cyanide for various orientations and distances of the methyl group with respect to the CN bond.
Volume $5, number I
CHEMICAL
PHYSICS
Fig. 3. Ener_gy relative to that of trifluoromethyl cyanide for various orientations and distances of the trifluoromethyl group with respect to the CN bond.
The results of both experiments and previous calculations are available for the barrier height or activation energy of isomerization of methy cyanide. Values of 40.8,34.3,32.9, and 58.8 k&/mole have been calculated by extended Hiickel [ 161, MIND0 [ 171, CNDO [ 181, and ab initio SCF [ 121 calculations, as compared with the experimental value of 38.4 kcal/ mole [ 191. The present value is approximately double that obtained experimentally. Liskow et al. [ 121 suggest that the correlation energy in the transition state for methyl cycnide would be larger than that for either the cyanide or isocyanide itself. Consequently the advantageous cancellation of errors which occurred in obtaining the energy of isomerization probably cannot be expected in calculating the ener,T of isomerization. Further, it is reasonable to assume that the calculated activation energy for trifluoromethyl cyanide is also too large. However, since it may be presumed that the correlation energy-of the transition state species in this case is higher than that with MeCN, the actual en-
LETTERS
1 April 1978
ergy of activation may well be considerably less than the value 38.4 kcal/mole as obtained experimentally for MeCN. The energy shifts of the highest occupied and lowest unoccupied moie&lar orbitals for the formation of the transition state are of interest and are included in fig. 1. The most striking feature of the transition state appears in the similarity of its ‘IIlevels to those of either the cyanide or isocyanide, contrasted with the substantially higher u and lower u* levels in the transition state than in either the cyanide or isocyanide. The overlap populations (table 3) provide information on the nature of the transition state. The x overlap populations suggest the existence of some Xbonding between the carbon of the methyl or trifiuoromethyl group and both the fixed carbon and nitrogen atoms, so that there may be some justification for referring to the transition state species as a n-complex. The fmed CN bond in the transition state displays a 71 overlap population less than that found in the same bond on either the cyanide or isocyanide. However, the G overlap population in the same bond is quite similar to that in the CN bond of the cyanides. Apparently the o bond in the CN group remains relatively unperturbed, while the ‘ITbond in that group becomes somewhat weakened, during the isomerizatioh process, the latter as a result of the shift of some density from the CN bonding region into the space between the methyl or TFMgroup and the CN group. The net atomic charge found on the nitrogen atom of the transition state species is the same for both the methyl and TFM molecules and is almost equaI to the charge on the nitrogen atom in methyl cyanide. The charge on the carbon atom of the CN group in the transition state species is slightly negative, in contrast to the positive charge found on that atom in both the cyanides and the isocyanides. However the ionic character in the transition state can be represented as [Me*o-208 ] [CN- o-2o8] for both methyl and TFM molecules, which is similar to the ionic character possessed by both isocyanides.
Acknowledgement The financial support of the National Research Council of Canada and the assistance and cooperation of the University of Waterloo Computing Centre are gratefully acknowledged. 129
Volume 55, number 1
CHEMICAL
PHYSICS LETTERS
References [I] J.B. Moffat, J_ Phyr Chem 81 (1977) 82. [Z] J. Lee and B-G. Wihou&by, Spectrochim. Acta-33A (1977) 395. [ 31 S-P_ Makarov, MA. Englin, A.F. Videiko and J.V. Niiolaeva, J. Gen. Chem. USSR 37 (1967) 2667. f4] R.E. Banks, RN. Haseldine, M.J. Stevenson and B-G. Willoughby, J. Chem. Sot. C (1969) 2119. [S] W.F. EdgeU and R-M. Potter, J. Chem. Phys. 24 (1956) [6] :.fk F aniran. H.F. ShurveU and S.!. Cyviu, J. Mot stnlct 10 (1971) 49. [7] R.L. Wiims, J. Chen Phys. 25 (1956) 656. [S] H-W. Thompson and RL. Wiims, Trans. Faraday Sot. 48 (19S2) 502_ [9) W-J. Hehre, W.A. Lathan. R. Ditchfield, M.D. Newton and J-k Pople, Gaussian 70, Quantum Chemistry Pro-
130
[lo] [ll] [12] [ 131 [ 141 [IS] [16] [ 171 [18] [19]
.I April 1978
=
gram Exchange, Cheniistry Department,-Indiana Univer: sity, Bloommgton, Indian% CC. Costain, J. Chem. Phya 29 (1958) 864, S.\V_Benson, I. Chem. Edtic. 42 (1965) 502. D.JL Liskow. CF. Bender and H.F.Sch&fer, J. Am. Chen Sot. 94 (1972) 5178. M.H. Baghal-Vayjooee, J.L. Collister and H-0. Pritchard, Can. J. Chem. 55 (1977) 2634. J.B. Moffat, Can. J. Chem., to be published. I. Schander atid B.R Russel!, 1. MoL Spectry. 65 (1977) 379, and references therein G.W. van Dine and R Hoffmann, J. Am. Chem. Sot. 90 (1968) 3227. M-J-S. Dewar and UC. Kahn, J. Am Chem. Sot. 94 (1972) 2704. J.B. Moffat a& K.F. Tang, Theoret. Chhn. Acta 32 (1973) 171. F.W. Schneider and B.S. Rabinovitch, J; Am Chem. Sot. 84 (1962) 4215.
A COWARATIVE AND ISOCYANIDES
CHEMICAL
THEORETICAL
PHYSICS
1 April 1978
LETTERS
STUDY OF TRIFLUOROMETHYL
AND THE CYANIDE-ISOCYANIDE
AND METHYL CYANIDES
ISOMERIZATION
PROCESS
J.B. MOFFAT Departntettt of Citetttistry and Gttelph-Waterloo Waterloo. Ontario. Canada NZI. 3GI Received
15 November
Certtre for Graduarc
Work in Chemistry.
iJttinmit_v
of Waterloo.
1977
Ab initio (STO-3G) geometry-optimized calctdations have been performed on methyl and trifluoromethyl cyanide and isocyanide, and energies of isomerization of 24.1 and 11.5 kcal/mole, respectively have been calculated. Activation barriers of 87.8 and 80.0 kwl/mole are predicted for the isomerization of methyl and tritluoromethyl cyanide, respectively, by esamining various features of the potential surface. The geometries, atomic charges, overlap populations, and molecular orbitals are employedto compare the two cyanides, isocyanides and transition species.
1. Introduction The structural isomerizations of molecules are important processes for both experimental and theoretical study. Because of the relative simplicity of the reacting system, extensive experimental data is obtainable, at least in principle, and a variety of theoretical techniques can be applied to provide desirable ancillary information. It is useful, as well as interesting, to examine systems where little or no experimental or theoretical information is available_ Here theoretical studies can hopefully assume-a predictive role for the experimentalist. The cyanide-isocyanide isomerization is an example of a unimolecular process where sufficient experimental and theoretical work has been performed on several cyanide-isocyanide systems, for example, methyl cyanide-isocyanide, to provide some insight into relative stabilities, possible isomerization mechanisms, rates, activation energies and related quantities. One of the systems for which somewhat less information is available is that of vinyl cyanide-isocyanide. Recent calculations have predicted differences in energy of 2 1.4 and 17.7 kcal/mole with an STO-3G and a 6-3 iG basis set, respectively, and energy barriers from the cyanide and isocyanide of 74.5 and 53.0 kcal/ mole, respectively ]I].
Recently, Lee and Willoughby [2] have reported the results of *.arious spectroscopic studies of trifluoromethyl isocyanide. This compound was first prepared ten years ago 13,4], and has been fo&d to be easily polymerized and isomerized to the cyanide. The vibrational spectra of related compounds trifluoromethyl cyanide [5,6], methyl isocyanide [7,8] and methyl cyanide [S] have been reported previously. Since no quantitative experimental information is available on the relative energies or rates of isomerization in the case of tritluoromethyl cyanide-isocyanide, but both isomers have been prepared, this appears to be a suitable system for predictive theoretical examination. In addition, the availability of experimental data for methyl cyanide and isocyanide provides opportunity for interesting comparisons. Ab initio SCF calculations [9] employing the STO3C basis and standard exponents were employed throughout. Geometry optimization of all bond lengt!rs and angles to 0.001 A and O-lo, respectively, was performed on both the cyanide and isocyanide. The CCN and CNC bonds were both held at 180”. An approximate energy surface for isomerization was constructed by fixed the CN bond on the x axis with the centre of the bond at the origin and allowing the trifluoromethyl group to move away from the CN bond along various paths perpendicular to that bond. Additional 125
Volume 55, nuniber 1
CHEMICAL PHYSICS LETTERS
1 April 1978
points on the potential surface were obtained by fir-
tances in the cyanides differ by almost 0.05 A, while
ing the trifluoromethyl group at a particular distance from the origin with the line joining the origin to the -CF, group passing through the N atom. This line is then rotated about the origin until it passes ‘through the carbon atom 180° later. Energies are calculated at selected intervals. For comparison purposes, similar calculations were performed on methyl cyanide and isocyanide.
the C-N lengths in the isocyanides differ by 0.018 A.
2. Results and
Somewhat surprisingly the bond angles in the two cyanides are quite similar, as are those in the two isocyanides.
The cyanides are predicted to be, as expected, more stable than the isocyanides (table 1). However a remarkable difference in the energies of isomerization for the two cyanides is observed, that for methyl cyanide (24.1 kcallmole) being more than twice as large as that for trifluoromethyl cyanide (11.5 kcal/mole). No experimental data are available for the energy of isomerization of trifhtoromethyl cyanide. Benson [ 1 l] estimated an energy difference of 15 kcal/mole between methyl cyanide and isocyanide. Liskow et al. 1121 found a value of 17.4 k&/mole from ab initio calculations. Very recently, Pritchard and co-workers 1131 have reported an enthalpy of isomerization of 23.7 + 0.14 kcal/mole for the isomerization of methyl cyanide to isocyanide. The present calculated valne of 24.1 kcal/mole is in surprisingly good agreement with
discussion
2. I. csliculated geometries isocyanides
of the cyanides
of isomerikation
2.2. Energies
and
The optimized nuclear configurations for trifluoromethyl cyanide and isocyanide are summarized in table I together with those for methyl cyanide and isocyanide for comparison. No experimental structural data are available for the former two molecules. The cyan0 bond lengths and isocyano bond lengths are slightly smaller in the methyl molecules compared to those of trifluoromethyl. However, the C-C dis-
Table 1 The optimized nuclear configurations and total electronic energies (hrcludmg nuclear repulsion) of trifluoromethyl cyanide and isocyanide F3CCN bond lengths (A) (iso) cyan0 CN cc
1.534
CN
-
xc a)
1.371
bond angles (deg) LXCC a) LXCN a)
~xcx a) electronic energy (hartree) AE
126
1.156
110.2
108.7
i13CCN
1.154 (1.157) b) 1.486 (1.458) 1.088 (1.103)
F3CNC
1.178 1.464 1.367
H3CNC
1.171 (1.166) 1.446 (1.424) 1.092 (1.101)
110.1 (109.5)
108.9
-422.63651
-130.27155
11.5
24.1
b) Values in parenthesesare experimental data from ref. [lo].
109.7 109.2 -422.61817
109.9 (109.1) 109.1 -130.23319
1 April 1978
CHEHICAL PHYSICSLETTERS
Volume 55, number 1
the experimental value of Pritchard. As Liskow et al.
[Me+“‘341
[ 121 point out, the correlation energies of MeCN and
for the isocyanides. Previous calculations [l] on vinyl cyanide and isocyanide predicted ionic characters represented by [C,Hp258] [CN-0*‘58] and [C,H3i0*4gg [NC-“-4gg], respectively. The ionic character& each of the three isocyanides is approximately double that found for the corresponding cyanide. However, the overlap populations for the nitrile bond are almost identical in the methyl and TFM molecules. In contrast, the o overlap population for the isocyanide bond is considerably larger in methyl isocyanide, while the ‘IToverlap populations are similar in both isocyanides. This suggests that the fluorine atom is capable of perturbing the u density without altering that of the z electrons.
MeNC are presumably approximately equal, and the differences of the calculated electronic energies from the exact single-determinant (Hartree-Fock) values may also be nearly the same [ 141, so that the present value for the energy difference is expected to be relatively independent of the basis set and hence reasonably reliable. In this regard it may be noted that the energy difference for vinyl cyanide-isocyanide was previously calculated ]l] as 21.4 and 17.7 kcal/mole with an STO-3G and a 6-3 1G basis set, respectively.
2-3. Charges Qtzdpopul~n~ons
[NC-0.2341
and [TFM+0.204 ] [NC-0.20?]
The net atomic charges and overlap populations are summarized in tables 2 and 3. The nitrile nitrogen
2.4. Molecular orbitals of the c_vanidesand isocyanides
atom in methyl cyanide is considerably more negative
As expected, the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of both methyl and trifluoromethyl cyanide are of approximately 1~symmetry (fig. 1). In the cyanides the orbitals immediately below the HOMO are of o symmetry_ In the isocyanides the LUMO are
than that in trifluoromethyl cyanide while the isonitrile carbon in the former molecule is much less positive than that in the latter. The ionic character can be represented as [Me+“-i23] [CN-u-1*3] and [TFM+0-0g7] [CN- uo97] for the cyanides and Table 2 Net atomic charges
methyl TFM C) a)X=ForH.
Transition state
isocyanide
Cyanide x a)
Cd)
C
N b)
x a)
c d)
N
c b)
x a)
cd)
N
C
+0.101 -0.122
-0.181 +0.464
+0.075 i0.041
-0.198 -0.138
+0.099 -0.122
-0.062 +0.568
-0.332 -0.37 1
+0.098 +0.167
+0.112 -0.099
-0.135 +0.504
-0.196 -0.196
-0.012 -0.010
b) Terminal atom.
C) TFM = trithroromethyl.
d) C attached to X.
Table 3 Overlap populations Cyanide
methyl TFM b)
a) Terminal atom. d, Sum of *p,-2px
Isocyanide
Transition state
cc
CN a)
CN
NC aI
c ck
C C)N
CN
0.170 0.0090 0.154 0.0108
0.098 0.444
0.157 0.0054
0.226 0.388
-0.010 +0.107
0.053
==0
0.440
0.139
0.372
+0.092
-0.015 +0.053 -0.01 ltO.O4?
0.083 0.333 0.092 0.333
0.095
0.0195
b) TFM = tritiuoromethyl. C) Carbon of methyl or tritluoromethyl group. and *py-*p,, overlap populations.
127
Volume 55, number 1
CHEMICAL
PHYSICS
3
+Q50 +Q40 +a30
+ozo
: F F;C-GIN F
-038
-
F F\-C-N F’
%C
1
Fig. 1. LUJfO and HOMO energies for cyanides, and transition state species.
1 April 1978
2.5. The transitiDrtstate The results of calculations in which the methyl or trifluoromethyl group orbit about the centre of the CN bond are shown in figs. 2 and 3. The abscissa represents the angle (8) made by the CN bond and the line (R) joining its centre and the carbon atom of either the methyl or trifluoromethyl group. Values of 8 of 0” and 180” represent the isocyanide and cyanide, respectively. Each line in f&s_ 2 and 3 traces the energy changes, with respect to the cyanide, and for a given fied length of the line R as the isornerization from isocyanide to cyanide, or vice versa, occurs. The similarity of the two graphs is readily evident. Two minima as well as the maximum, the latter representing the activation barrier, can be clearly seen. The minimum barrier for the MeCN-MeNC isomerization occurs at an R value of 1.70 A, a 6 value of loo’, and an energy change of 87.8 kcal/mole. For the F,CCNF,CNC isomerization, the minimum barrier of 80 kcal/mole occurs for an R value of 1.76 a at 100”.
isocyanides,
again of rr symmetry but now the HOMO are of o symmetry and the orbitals immediately below the HOMO are of o symmetry_ As fig. 1 illustrates, isomerization of either cyanide to the corresponding isocyanide involves a small increase of the LUMO (T*) energy and an even smaller decrease in the energy of the occupied rr orbital. However the energy of the occupied u orbital increases substantially and crosses that of the occupied 7~orbiial so that the occupied u orbital becomes the HOMO in the isocyanides. It should also be noted that substitution of fluorine for hydrogen in the isocyanide produces a stabilization of the highest occupied o orbital, but an even greater decrease in energy of the highest occupied of orbital. Most noticeable however is the marked x-stabilization on substitution of fluorine for hydrogen in the cyanide, while the u orbital remains almost unchanged in energy- This appears to be the reverse of the so-called perfluoro effect which has been observed in planar ethylenic molecules [ 151.
128
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0
I
zo
I 40
I 80
I
80 8
I
loo (deg.1
I
I
I20
I40
Ia3
I
Fig. 2. Energy relative to that of methyl cyanide for various orientations and distances of the methyl group with respect to the CN bond.
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Fig. 3. Ener_gy relative to that of trifluoromethyl cyanide for various orientations and distances of the trifluoromethyl group with respect to the CN bond.
The results of both experiments and previous calculations are available for the barrier height or activation energy of isomerization of methy cyanide. Values of 40.8,34.3,32.9, and 58.8 k&/mole have been calculated by extended Hiickel [ 161, MIND0 [ 171, CNDO [ 181, and ab initio SCF [ 121 calculations, as compared with the experimental value of 38.4 kcal/ mole [ 191. The present value is approximately double that obtained experimentally. Liskow et al. [ 121 suggest that the correlation energy in the transition state for methyl cycnide would be larger than that for either the cyanide or isocyanide itself. Consequently the advantageous cancellation of errors which occurred in obtaining the energy of isomerization probably cannot be expected in calculating the ener,T of isomerization. Further, it is reasonable to assume that the calculated activation energy for trifluoromethyl cyanide is also too large. However, since it may be presumed that the correlation energy-of the transition state species in this case is higher than that with MeCN, the actual en-
LETTERS
1 April 1978
ergy of activation may well be considerably less than the value 38.4 kcal/mole as obtained experimentally for MeCN. The energy shifts of the highest occupied and lowest unoccupied moie&lar orbitals for the formation of the transition state are of interest and are included in fig. 1. The most striking feature of the transition state appears in the similarity of its ‘IIlevels to those of either the cyanide or isocyanide, contrasted with the substantially higher u and lower u* levels in the transition state than in either the cyanide or isocyanide. The overlap populations (table 3) provide information on the nature of the transition state. The x overlap populations suggest the existence of some Xbonding between the carbon of the methyl or trifiuoromethyl group and both the fixed carbon and nitrogen atoms, so that there may be some justification for referring to the transition state species as a n-complex. The fmed CN bond in the transition state displays a 71 overlap population less than that found in the same bond on either the cyanide or isocyanide. However, the G overlap population in the same bond is quite similar to that in the CN bond of the cyanides. Apparently the o bond in the CN group remains relatively unperturbed, while the ‘ITbond in that group becomes somewhat weakened, during the isomerizatioh process, the latter as a result of the shift of some density from the CN bonding region into the space between the methyl or TFMgroup and the CN group. The net atomic charge found on the nitrogen atom of the transition state species is the same for both the methyl and TFM molecules and is almost equaI to the charge on the nitrogen atom in methyl cyanide. The charge on the carbon atom of the CN group in the transition state species is slightly negative, in contrast to the positive charge found on that atom in both the cyanides and the isocyanides. However the ionic character in the transition state can be represented as [Me*o-208 ] [CN- o-2o8] for both methyl and TFM molecules, which is similar to the ionic character possessed by both isocyanides.
Acknowledgement The financial support of the National Research Council of Canada and the assistance and cooperation of the University of Waterloo Computing Centre are gratefully acknowledged. 129
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References [I] J.B. Moffat, J_ Phyr Chem 81 (1977) 82. [Z] J. Lee and B-G. Wihou&by, Spectrochim. Acta-33A (1977) 395. [ 31 S-P_ Makarov, MA. Englin, A.F. Videiko and J.V. Niiolaeva, J. Gen. Chem. USSR 37 (1967) 2667. f4] R.E. Banks, RN. Haseldine, M.J. Stevenson and B-G. Willoughby, J. Chem. Sot. C (1969) 2119. [S] W.F. EdgeU and R-M. Potter, J. Chem. Phys. 24 (1956) [6] :.fk F aniran. H.F. ShurveU and S.!. Cyviu, J. Mot stnlct 10 (1971) 49. [7] R.L. Wiims, J. Chen Phys. 25 (1956) 656. [S] H-W. Thompson and RL. Wiims, Trans. Faraday Sot. 48 (19S2) 502_ [9) W-J. Hehre, W.A. Lathan. R. Ditchfield, M.D. Newton and J-k Pople, Gaussian 70, Quantum Chemistry Pro-
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[lo] [ll] [12] [ 131 [ 141 [IS] [16] [ 171 [18] [19]
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=
gram Exchange, Cheniistry Department,-Indiana Univer: sity, Bloommgton, Indian% CC. Costain, J. Chem. Phya 29 (1958) 864, S.\V_Benson, I. Chem. Edtic. 42 (1965) 502. D.JL Liskow. CF. Bender and H.F.Sch&fer, J. Am. Chen Sot. 94 (1972) 5178. M.H. Baghal-Vayjooee, J.L. Collister and H-0. Pritchard, Can. J. Chem. 55 (1977) 2634. J.B. Moffat, Can. J. Chem., to be published. I. Schander atid B.R Russel!, 1. MoL Spectry. 65 (1977) 379, and references therein G.W. van Dine and R Hoffmann, J. Am. Chem. Sot. 90 (1968) 3227. M-J-S. Dewar and UC. Kahn, J. Am Chem. Sot. 94 (1972) 2704. J.B. Moffat a& K.F. Tang, Theoret. Chhn. Acta 32 (1973) 171. F.W. Schneider and B.S. Rabinovitch, J; Am Chem. Sot. 84 (1962) 4215.