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Ab initio study of the conformations of chloromethyl formate and fluoromethyl formate
Journal of Molecular Structure, 63 (1980) 287-292 0 Elsevier Scientific Publishing Company, Amsterdam
-
Printed in The Netherlands
AR INITIO STUDY OF THE CONFORMATIONS FORMATE AND FLUOROMETHYL FORMATE
MATTI
OF CHLOROMETHYL
I-IOTOKKA
Department MARTTI
of Physical Chemistry,
A bo Akademi,
SF-20500
A bo 50 (Finland)
G. DAHLQVIST
Department (Finland) (Received
of Chemistry
and Biochemistry,
12 October 1978;
University of Turku, SF-20500
Turku 50
in final form 18 April 1979)
ABSTRACT Ab initio calculations are reported for the conformational potential energy surfaces of chloromethyl formate and fluoromethyl formate at minimal basis set level. The halomethyl group is shown to lie 2 to the carbonyl group. A plateau on the potential energy surface demonstrates that the halomethyl group can rotate freely. The halogen atom therefore moves from an antiperiplanar (up) to a synclinal (sp) position with respect to the carbonyl group. The effects on IR spectra and dipole moments are discussed.
INTRODUCTION
Chloromethyl formate has been used for some years as a model for studying structural and polar effects in acid- and base-catalysed ester hydrolysis [ 1, 21. The structure of methyl formate has been extensively studied, both theoretically [3-1551 and experimentally [16,17], but the halomethyl formates are not as well known. The model system for halomethyl esters has usually been chloromethyl acetate [16, IS]. Recently the conformations of chloromethyl formate have been studied using IR and theoretical CNDO/B
techniques [ 191. In contrast to the IR results, the semiempirical calculations suggested that the chlorine atom should lie synplanar (sp) to the carbonyl ’ group. (The nomenclature suggested by Dahlqvist and Euranto [20] is used throughout this paper). Since this result was obviously incorrect we decided to study the conformational potential energy surfaces of chloromethyl formate and fluoromethyl formate using ab initio methods. CALCULATIONS
The ab initio MO-LCAO-SCF program MOLECULE 1211 was used. For hydrogen we chose a (3s) set of Gaussian-type functions [22], while for the first row atoms, carbon, oxygen and fluorine, we used a (7s, 3~) set [ 231 and for chlorine a (lOs, 6~) set [23] of primitive Gaussians, all contracted to
288
HH Fig. 1. The geometry of the halomethyl formates (X = F or CI). The angles CY,, cyz and fl are varied while the others are fixed at the values of sp2 and sp3 hybrids. The bond distances are R(C-F) 133 pm and R(C-Cl) 178 pm. The distances in the figure (in pm) are taken from refs. 8 and 9. E(a u.1 -325Ll
,325 l-6 325 LI
-325
L6
.325Ll
-32516 - 325.41
- 325-10
Fig. 2. Cross-sections of formate and fluoromethyl
the conformational potential energy surfaces for chloromethyl formate. 01, = 0”. (0.01 hartree = 26.25 kJ mol-‘).
minimal basis sets. In order to confirm the reliability of the results, the general shape of the potential energy surface and the p angle of fluoromethyl
289
formate were calculated using Dunning’s double-zeta contraction [25] of Huzinaga’s (9s, 5~) basis set [ 221. All the computations were performed on the Finnish Universities’ UNIVAC 1108. The geometries considered are shown in Fig. 1. The total energies of both molecules were computed at 18 points in order to study that part of the conformational potential energy surface where Q! , is zero. In addition, the optimal values for cy2and p were obtained in the case cy.,= 180’ for fluoromethyl formate, the same geometry also being assumed valid for chloromethyl formate. Cross-sections of the potential energy surfaces are shown in Fig, 2, and the computed dipole moments are given in Tables 1 and 2. The MulIiken net and overlap populations are given in Fig. 3, and the double-zeta calculations are summarized in Table 3. DISCUSSION
A major part of the conformational potential energy surfaces for chloromethyl formate and fluoromethyl fomate is very flat. The cross-sections shown in Fig. 2 clearly indicate that the halomethyl group can rotate freely from ar23 60” to CY~= 300”. This is contradictory to the previous CNDO/B calculations 1191 which gave a minimum at cy2* 0” and weak net bonding between the hafogen atom and the carbonyl group. It was recently proposed, on the basis of IR and dipole moment data 1183 , that c~orom~thyl acetate would have a single, well-defined conformation, namely the 2, ap form. Substitution of acetate for formate should have little effect on the potential seen by the chloromethyl group. Therefore, essentialfy unhindered rotation would be~xpe~~dbetween~=~60°~d~2- 300" in both the acetate and formate. The experiment dipole moment of chloromethyl acetate [18],2.23 L), is quite close to the present value for chloromethyl formate near the classical turning point, with 0~~close to 60” (Table 2). This is the most likely conformation of the molecule. It has been well established experimentally that for simple carboxylic esters the skeleton C(O)OC is planar, or nearly so, with the O-C bond in a 2 orientation with respect to the carbonyl group [ 11,16,17] . It is also found theoretically that the halomethyl group is 2 to the carbonyl group, i.e. the value of angle ar, is predicted to be 0”. The halogen atom itself points away from the carbonyl group. The minimal basis set calculations suggested that the angle /3is roughly 125”. Calculations on methyl formate with the same basis set yielded an oxygen bond angle of about 120°. The double-zeta calculations predict the angle fi to be 122” in fluoromethyl formate, in good agreement with the minimal-basis results. This represents an increase of about 10” from the values for dimethyl ether (110”) [24] or for methyl formate (115”) [9]_ It is interesting to speculate on why an electronegative substituent on carbon should open the C-O-C angle. According to the minimal-basis calculations, the energy of the E, up conformation with OL,= ISO”, 0~~= 180” and 0 = 122” is 15.7 and 16.8 kJ
290 TABLE
1
Computed
(de& 0
0
60
120
110 120 130 140
1.90 2.37 2.71 2.99
2.13 2.26 2.45 2.62
1.57 1.44 1.44 1.53
-
-
-
0
180
dipole moments for chloromethyl
(deg)
180
0.95 0.82
1.03 0.69 0.42
-
-
*
3.64
formate (D)
P
a, (deg)
(deg)
0
60
120
110 120 130 140 125
2.04 2.24 2.61 2.96
2.11 2.31 2.51 2.68
1.68 1.49 1.47 1.55
-
-
-
150
180
0.94 0.80
1.04 0.66 0.39 0.31 3.6 1
-
-
3
Results of the caIcuIations with the double-zeta aI
150
2
Computed
TABLE
formate (D)
(deg)
125
180 TABLE
dipole moments for fluoromethyl
a2
(deg)
basis set [25 J for fluoromethyl
formate
P (de@
0
180
110 115 120 125 130
-326.56138 -326.56682 -326.56893 -326.56884 -326.56673
0
0 60 120
120
-326-52626 -326.56647 -326.56720
mol-’ above the energy minima for chloromethyl formate and ~uoromethyl formate, respectively. We may estimate that the energy of a molecule with dipole moment ~1and radius r. is lowered by &7=&L e-1 (1) r’o 2E + 1 in a solvent with dielectric constant E. Choosing the values E = 36 and r. = 300 pm we obtain an order-of-magnitude estimate of 10 kJ mol-’ for the energy lowering of the E, up conformation relative to the 2, ap form. It is therefore possible that the E, up conformation might be detected in very polar solvents,
291 0.45
IO.LS)
H 0.79
10.79) \
4.63 fL.621
1685
297
i3.11r
11971
C 0.99
J 0
(0.99
7.95
(7.951
t-tH O.&Q
0.49
(0.521 10.521
Fig. 3. The Mulliken net and overlap populations of chloromethyl formate and fiuoromethyl formate (in parentheses) for the case: OL,= O”, u2 = 180” and p = 120*.
and there is actually IR spectroscopic evidence [ 193 for the existence of the E, ap form of chloromethyl formate in acetonitrile solution. ACKNOWLEDGEMENTS Financial support from the Academy of Finland is gratefully acknowledged. We also thank Professor P. Pyykkij for usefuI discussions. REFERENCES 1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17 18 19 20
E. K. Euranto and R. Euranto, Suom. Kemistil., B, 35 (1962) 96. E. K. Euranto, Ann. Univ. Turku., Ser. A, 42 (1960) 20. EL.V. Cheney and R. E. Christoffersen, J. Chem. Phys., 56 (1972) 3503. H. WennerstrSm, S. Forsen and B. ROOS, J. Phys. Chem., 76 (1972) 2430. L. Radom. W. A. Lathan, W. J- Hehre and J. A. Pople, Aust. J. Chem., 25 (1972) 1601. M. Perricaudet and A. Pullman, Int. J. Pept. Protein Res., 5 (1973) 99. D. G. Lister, N. L. Owen and P. Palmieri, J. Mol. Struct., 31 (19’76) 411. D. G. Lister and P. Palmieri, J. Mol. Struct., 32 (1976) 355. G. I. L. Jones, I). G. Lister, N. L. Owen, M. C. L. Gerry and P. Palmieri, J. Mol. Spectrosc., 60 (1976) 348. I. G. John and L. Radom, J. Mol. Struct., 36 (1977) 133. H. Susi and J. R. Scherer, Spectrochim. Acta, Part A, 25 (1969) 1243. Y. Ono and Y. Ueda, Chem. Pharm. Bull., 18 (1970) 2013. N. Bodor and N. Trinajstic, Rev. Roum. Chim., 15 (1970) 1807. M. T. Meaume, S. Odiot and N. Brigot, C. R. Acad. Sci., Ser. B, 272 (1971) 591. P. Matzke and F. Peradejordi, C. R. Acad. Sci., Ser. D, 282 (1976) 321. G, I. L. Jones and N. L. Owen, J. Mol. Struct., 18 (1973) 1. * R. F. Curl, Jr., J. Chem. Phys., 30 (1959) 1529. S. W- Charles, G. I. L. Jones, N. L. Owen and L. A_ West, J. Mol. Struct., 32 (1976) 111. M. G. Dahlq~~, Spectrochim. Acta, Part A, in press. M. G. Dahlqvist and E. K. Eumnto, Spectrochim. Acta, Part A, 34 (1978) 863_
292 21 J. AlmiX,
22 23 24 25
University of Stockholm, Institute of Theoretical Physics, Rep. No. 72-09 (Sept. 1972) and No. 74-29 (Dec. 19’74). 8. Huzinaga, J- Chem. Phys., 42 (1965) 1293. B. Roos and P. Siegbahn, Theor. Chim, Acta, 17 (1970) 209. Handbook of Chemistry and Physics, 57th edn., C.R.C. press, Cieveland, OH, 19751976. T. H. Dunning, Jr., J. Chem. Phys., 53 (1970) 2823.
-
Printed in The Netherlands
AR INITIO STUDY OF THE CONFORMATIONS FORMATE AND FLUOROMETHYL FORMATE
MATTI
OF CHLOROMETHYL
I-IOTOKKA
Department MARTTI
of Physical Chemistry,
A bo Akademi,
SF-20500
A bo 50 (Finland)
G. DAHLQVIST
Department (Finland) (Received
of Chemistry
and Biochemistry,
12 October 1978;
University of Turku, SF-20500
Turku 50
in final form 18 April 1979)
ABSTRACT Ab initio calculations are reported for the conformational potential energy surfaces of chloromethyl formate and fluoromethyl formate at minimal basis set level. The halomethyl group is shown to lie 2 to the carbonyl group. A plateau on the potential energy surface demonstrates that the halomethyl group can rotate freely. The halogen atom therefore moves from an antiperiplanar (up) to a synclinal (sp) position with respect to the carbonyl group. The effects on IR spectra and dipole moments are discussed.
INTRODUCTION
Chloromethyl formate has been used for some years as a model for studying structural and polar effects in acid- and base-catalysed ester hydrolysis [ 1, 21. The structure of methyl formate has been extensively studied, both theoretically [3-1551 and experimentally [16,17], but the halomethyl formates are not as well known. The model system for halomethyl esters has usually been chloromethyl acetate [16, IS]. Recently the conformations of chloromethyl formate have been studied using IR and theoretical CNDO/B
techniques [ 191. In contrast to the IR results, the semiempirical calculations suggested that the chlorine atom should lie synplanar (sp) to the carbonyl ’ group. (The nomenclature suggested by Dahlqvist and Euranto [20] is used throughout this paper). Since this result was obviously incorrect we decided to study the conformational potential energy surfaces of chloromethyl formate and fluoromethyl formate using ab initio methods. CALCULATIONS
The ab initio MO-LCAO-SCF program MOLECULE 1211 was used. For hydrogen we chose a (3s) set of Gaussian-type functions [22], while for the first row atoms, carbon, oxygen and fluorine, we used a (7s, 3~) set [ 231 and for chlorine a (lOs, 6~) set [23] of primitive Gaussians, all contracted to
288
HH Fig. 1. The geometry of the halomethyl formates (X = F or CI). The angles CY,, cyz and fl are varied while the others are fixed at the values of sp2 and sp3 hybrids. The bond distances are R(C-F) 133 pm and R(C-Cl) 178 pm. The distances in the figure (in pm) are taken from refs. 8 and 9. E(a u.1 -325Ll
,325 l-6 325 LI
-325
L6
.325Ll
-32516 - 325.41
- 325-10
Fig. 2. Cross-sections of formate and fluoromethyl
the conformational potential energy surfaces for chloromethyl formate. 01, = 0”. (0.01 hartree = 26.25 kJ mol-‘).
minimal basis sets. In order to confirm the reliability of the results, the general shape of the potential energy surface and the p angle of fluoromethyl
289
formate were calculated using Dunning’s double-zeta contraction [25] of Huzinaga’s (9s, 5~) basis set [ 221. All the computations were performed on the Finnish Universities’ UNIVAC 1108. The geometries considered are shown in Fig. 1. The total energies of both molecules were computed at 18 points in order to study that part of the conformational potential energy surface where Q! , is zero. In addition, the optimal values for cy2and p were obtained in the case cy.,= 180’ for fluoromethyl formate, the same geometry also being assumed valid for chloromethyl formate. Cross-sections of the potential energy surfaces are shown in Fig, 2, and the computed dipole moments are given in Tables 1 and 2. The MulIiken net and overlap populations are given in Fig. 3, and the double-zeta calculations are summarized in Table 3. DISCUSSION
A major part of the conformational potential energy surfaces for chloromethyl formate and fluoromethyl fomate is very flat. The cross-sections shown in Fig. 2 clearly indicate that the halomethyl group can rotate freely from ar23 60” to CY~= 300”. This is contradictory to the previous CNDO/B calculations 1191 which gave a minimum at cy2* 0” and weak net bonding between the hafogen atom and the carbonyl group. It was recently proposed, on the basis of IR and dipole moment data 1183 , that c~orom~thyl acetate would have a single, well-defined conformation, namely the 2, ap form. Substitution of acetate for formate should have little effect on the potential seen by the chloromethyl group. Therefore, essentialfy unhindered rotation would be~xpe~~dbetween~=~60°~d~2- 300" in both the acetate and formate. The experiment dipole moment of chloromethyl acetate [18],2.23 L), is quite close to the present value for chloromethyl formate near the classical turning point, with 0~~close to 60” (Table 2). This is the most likely conformation of the molecule. It has been well established experimentally that for simple carboxylic esters the skeleton C(O)OC is planar, or nearly so, with the O-C bond in a 2 orientation with respect to the carbonyl group [ 11,16,17] . It is also found theoretically that the halomethyl group is 2 to the carbonyl group, i.e. the value of angle ar, is predicted to be 0”. The halogen atom itself points away from the carbonyl group. The minimal basis set calculations suggested that the angle /3is roughly 125”. Calculations on methyl formate with the same basis set yielded an oxygen bond angle of about 120°. The double-zeta calculations predict the angle fi to be 122” in fluoromethyl formate, in good agreement with the minimal-basis results. This represents an increase of about 10” from the values for dimethyl ether (110”) [24] or for methyl formate (115”) [9]_ It is interesting to speculate on why an electronegative substituent on carbon should open the C-O-C angle. According to the minimal-basis calculations, the energy of the E, up conformation with OL,= ISO”, 0~~= 180” and 0 = 122” is 15.7 and 16.8 kJ
290 TABLE
1
Computed
(de& 0
0
60
120
110 120 130 140
1.90 2.37 2.71 2.99
2.13 2.26 2.45 2.62
1.57 1.44 1.44 1.53
-
-
-
0
180
dipole moments for chloromethyl
(deg)
180
0.95 0.82
1.03 0.69 0.42
-
-
*
3.64
formate (D)
P
a, (deg)
(deg)
0
60
120
110 120 130 140 125
2.04 2.24 2.61 2.96
2.11 2.31 2.51 2.68
1.68 1.49 1.47 1.55
-
-
-
150
180
0.94 0.80
1.04 0.66 0.39 0.31 3.6 1
-
-
3
Results of the caIcuIations with the double-zeta aI
150
2
Computed
TABLE
formate (D)
(deg)
125
180 TABLE
dipole moments for fluoromethyl
a2
(deg)
basis set [25 J for fluoromethyl
formate
P (de@
0
180
110 115 120 125 130
-326.56138 -326.56682 -326.56893 -326.56884 -326.56673
0
0 60 120
120
-326-52626 -326.56647 -326.56720
mol-’ above the energy minima for chloromethyl formate and ~uoromethyl formate, respectively. We may estimate that the energy of a molecule with dipole moment ~1and radius r. is lowered by &7=&L e-1 (1) r’o 2E + 1 in a solvent with dielectric constant E. Choosing the values E = 36 and r. = 300 pm we obtain an order-of-magnitude estimate of 10 kJ mol-’ for the energy lowering of the E, up conformation relative to the 2, ap form. It is therefore possible that the E, up conformation might be detected in very polar solvents,
291 0.45
IO.LS)
H 0.79
10.79) \
4.63 fL.621
1685
297
i3.11r
11971
C 0.99
J 0
(0.99
7.95
(7.951
t-tH O.&Q
0.49
(0.521 10.521
Fig. 3. The Mulliken net and overlap populations of chloromethyl formate and fiuoromethyl formate (in parentheses) for the case: OL,= O”, u2 = 180” and p = 120*.
and there is actually IR spectroscopic evidence [ 193 for the existence of the E, ap form of chloromethyl formate in acetonitrile solution. ACKNOWLEDGEMENTS Financial support from the Academy of Finland is gratefully acknowledged. We also thank Professor P. Pyykkij for usefuI discussions. REFERENCES 1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17 18 19 20
E. K. Euranto and R. Euranto, Suom. Kemistil., B, 35 (1962) 96. E. K. Euranto, Ann. Univ. Turku., Ser. A, 42 (1960) 20. EL.V. Cheney and R. E. Christoffersen, J. Chem. Phys., 56 (1972) 3503. H. WennerstrSm, S. Forsen and B. ROOS, J. Phys. Chem., 76 (1972) 2430. L. Radom. W. A. Lathan, W. J- Hehre and J. A. Pople, Aust. J. Chem., 25 (1972) 1601. M. Perricaudet and A. Pullman, Int. J. Pept. Protein Res., 5 (1973) 99. D. G. Lister, N. L. Owen and P. Palmieri, J. Mol. Struct., 31 (19’76) 411. D. G. Lister and P. Palmieri, J. Mol. Struct., 32 (1976) 355. G. I. L. Jones, I). G. Lister, N. L. Owen, M. C. L. Gerry and P. Palmieri, J. Mol. Spectrosc., 60 (1976) 348. I. G. John and L. Radom, J. Mol. Struct., 36 (1977) 133. H. Susi and J. R. Scherer, Spectrochim. Acta, Part A, 25 (1969) 1243. Y. Ono and Y. Ueda, Chem. Pharm. Bull., 18 (1970) 2013. N. Bodor and N. Trinajstic, Rev. Roum. Chim., 15 (1970) 1807. M. T. Meaume, S. Odiot and N. Brigot, C. R. Acad. Sci., Ser. B, 272 (1971) 591. P. Matzke and F. Peradejordi, C. R. Acad. Sci., Ser. D, 282 (1976) 321. G, I. L. Jones and N. L. Owen, J. Mol. Struct., 18 (1973) 1. * R. F. Curl, Jr., J. Chem. Phys., 30 (1959) 1529. S. W- Charles, G. I. L. Jones, N. L. Owen and L. A_ West, J. Mol. Struct., 32 (1976) 111. M. G. Dahlq~~, Spectrochim. Acta, Part A, in press. M. G. Dahlqvist and E. K. Eumnto, Spectrochim. Acta, Part A, 34 (1978) 863_
292 21 J. AlmiX,
22 23 24 25
University of Stockholm, Institute of Theoretical Physics, Rep. No. 72-09 (Sept. 1972) and No. 74-29 (Dec. 19’74). 8. Huzinaga, J- Chem. Phys., 42 (1965) 1293. B. Roos and P. Siegbahn, Theor. Chim, Acta, 17 (1970) 209. Handbook of Chemistry and Physics, 57th edn., C.R.C. press, Cieveland, OH, 19751976. T. H. Dunning, Jr., J. Chem. Phys., 53 (1970) 2823.