2782
J. Tyrrell, J. G. Ellison, and S. Stayton
The Journal of Physical Chemistry, Vol. 83, No. 21, 1979
Theoretical Investigation of Internal Rotation in Fluoro- and Chloroglyoxal James Tyrrell, Jeffrey G. Ellison, and Stephen Stayton Department of Chemistry and Biochemistry, Southern Illinois University at Carbondale, Carbondale, Illinois 6290 1 (Received May 3, 1979)
Semiempiricaland ab initio molecular orbital methods have been used to investigate internal rotation in fluoroand chloroglyoxal. Both molecules appear to exist as the trans and cis conformers, with the former conformer being the more abundant. The barrier to internal rotation from the trans conformer is 3.46 and 4.14 kcal/mol for fluoroglyoxal and chloroglyoxal, respectively. The four highest occupied molecular orbitals in order of increasing Koopman’s theorem ionization potential are n, T , n, T for fluoroglyoxaland n, n, n, n for chloroglyoxal.
Introduction There have been numerous investigations of the 1,2dicarbonyls OXCCXO using both experimental and theoretical techniques. The primary goal of these studies has been the determination of the number and percent abundance of the rotational conformers, the barrier to internal rotation, and the factors which contribute to the barrier. Another subject of investigation has been the nature and ordering of the valence molecular orbitals. This information aids in predicting the nature of the molecule’s electronic transitions and therefore contributes to our understanding of the photochemistry of these molecules. All of the simple 1,2-dicarbonylsexist predominantly in the trans conformation in their ground electronic state at room temperature. However, other possible conformers such as the cis or gauche conformer have been detected for certain molecules. Glyoxal (OHCCHO) has been shown both spectroscopically1i2and theoretically3 to exist in both the planar trans and the planar cis forms with a calculated trans-cis energy difference of 5.9 kcal/mo13 and a barrier to internal rotation from the trans conformer of 7.5 kcal/m01.~Spectroscopic4and theoretical5 techniques have also been used to show that oxalyl fluoride (OFCCFO) exists in both the trans and the cis forms. The calculated energy difference of the two conformers and the barrier to internal rotation is 0.54 and 5.23 kcal/mol, re~pectively.~ Oxalyl chlorofluoride (OCICCFO) has only been detected in the trans form spectroscopically,6 but theoretical calculations6 have indicated the probable presence of a cis conformer as well with a trans-cis energy difference of 0.28 kcal/mol and a barrier to internal rotation of 3.20 kcal/ mol, For oxalyl chloride (OClCCClO) the spectroscopic evidence is not entirely definitive, indicating either a trans and a cis conformer’ or a trans and a gauche conformer.s An ab initio investigation of oxalyl chloride5 supports the latter situation with a trans-gauche energy separation of 2.8 kcal/mol and a barrier of 3.32 kcal/mol. Biacetyl (CH3COCOCH3)has been found experimentallyg only as the trans conformer, and a theoretical investigationlo has shown that, while a minimum in the internal rotation potential about the central C-C bond is also present for the cis conformer, the energy separation is such that no appreciable amount of the cis conformer would be present at room temperature. Finally, an analysis of the microwave spectrum of methylglyoxal CH3COCH011only detected the trans conformer. No experimental data are available on either fluoroglyoxal (OFCCHO) or chloroglyoxal (OClCCHO). Ab initio investigations of g l y ~ x a l oxalyl ,~ fl~oride,~ oxalyl chlor~fluoride,~ and biacetyllO all indicate that the four highest occupied molecular orbitals are n, T , n, T in 0022-3654/79/2083-2782$01 .OO/O
order of increasing Koopman’s theorem ionization potential. In oxalyl chloride5 the ordering is n, n, T , 7, but the second and third orbitals are nearly degenerate and their ordering is not definitive. These results suggest the possibility of T T* transitions with energies similar to n ?r* transitions. In fact, Lucchese and Schaefer,12using the direct configuration interaction method, have found the 3 B u ( ~ - ~ *state ) of glyoxal to be somewhat lower in energy than the 3B,(n-7r*) state. This investigation uses both semiempirical and ab initio molecular orbital techniques to determine the optimum geometries, internal rotation potentials, and valence molecular orbital ordering in fluoro- and chloroglyoxal.
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Method and Results The semiempirical molecular orbital procedure programmed as MIND0/312 was used to determine optimum geometries and total energies for fluoroglyoxal at a number of dihedral angle values. These geometries and total energies, determined under the condition that all geometrical parameters except for the dihedral angles are unconstrained, are given in Table I. If the dihedral angles and the CCF angle are not allowed to vary, then a somewhat different picture of the fluoroglyoxal geometry as a function of dihedral angle is obtained, and this is shown in Table 11. Table I11 lists the total energies as a function of dihedral angle for fluoroglyoxal, derived from the GAUSSIAN 7o14 ab initio program and an STO-4G basis set. The results listed were obtained by treating the molecule as a rigid rotor and by using, in the first case, the ~ 1 ~ ~ 0 / 3 - o p t i m itrans z e d geometry and, in the second case, the same geometry modified by having the CCF and OCF angles optimized by an ab initio calculation. Finally, for fluoroglyoxal, Table IV lists the highest occupied molecular orbitals obtained by the two geometries previously described (Table 111). These orbitals were obtained by using the GAUSSIAN 70 program and the STO-4G basis set. Table IV also includes the orbital energy levels obtained when a double-{ basis set was used with the IBMOL 5A16 ab initio program for the trans conformer with the MINDO/ 3-optimized geometry. The basis set for the hydrogen was a (6S/2S) contracted Gaussian type and for the other atoms was a (13S, 7P/4S, 2P) contracted Gaussian type.16 The geometry used for chloroglyoxal is given in Table V and is essentially that of fluoroglyoxal except for the C-C1 bond length, where the value found for oxalyl chlorides is used. Table VI gives the values for the total energy of chloroglyoxal as a function of dihedral angle. These energies were obtained by using the GAUSSIAN 70 program and an STO-4G basis set with the molecule treated as a rigid rotor. Finally, Table VI1 lists the highest
0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 27, 7979 2703
Internal Rotation in Fluoro- and Chloroglyoxal
TABLE I: M1~DO/3-Optimized~ Geometry of Fluoroglyoxal as a Function of Dihedral Angle dihedral angle, deg
RC-F RC=O(F)
Rc-c RC-H
Rc=om.I)
L FCO L CCF
L CCH
L HCO Etot, eV R H. . . F
0
30
60
90
120
150
180
1.353 1.178 1.496 1.136 1.187 128.2 100.2 109.0 121.1 -1369.2524 3.198
1.350 1.177 1.490 1.137 1.188 130.7 95.4 108.2 121.3 -1369.3315 3.063
1.349 1.169 1.486 1.139 1.193 138.9 76.5 105.0 122.0 -1369.5667 2.555
1.366 1.165 1.489 1.131 1.197 138.7 71.9 108.9 123.5 -1369.7811 2.208
1.369 1.163 1.494 1.123 1.199 138.3 69.4 114.2 124.1 - 1369.8125 1.883
1.362 1.164 1.498 1.126 1.199 140.1 65.5 114.4 125.0 -1369.7653 1.531
1.371 1.168 1.485 1.152 1.199 139.7 63.1 108.8 126.5 -1369.8130 1.243
a All geometrical parameters except dihedral angles are optimized. Bond lengths are in angstroms and bond angles are in degrees.
TABLE 11: MINDO/3-Optimizeda Geometry of Fluoroglyoxal as a Function of Dihedral Angle dihedral angle, deg
0 RC-F RC=O(F)
Rc-c RC-H C=O(H)
LFCO L CCF L CCH LHCO Etot, eV RH . . . F
30
1.353 1.178 1.496 1.136 1.187 128.2 100.2 109.0 121.1 -1369.2524 3.198
60
1.353 1.179 1.490 1.135 1.188 128.3 100.2 109.2 121.6 -1369.3241 3.133
1.353 1.179 1.481 1.134 1.190 128.1 100.2 109.3 122.7 -1369.4599 2.956
90
120
1.355 1.178 1.476 1.132 1.191 127.7 100.2 110.1 123.3 - 1369.5406 2.709
1.357 1.178 1.479 1.128 1.192 126.7 100.2 112.0 123.1 - 1369.5172 2.463
150
180
1.356 1.178 1.485 1.127 1.192 125.9 100.2 112.6 123.1 -1369.4398 2.262
1.356 1.178 1.487 1.126 1.192 125.5 100.2 112.5 123.1 -1369.4016 2.179
a All geometrical parameters except dihedral angles and the LCCF are optimized. Bond lengths are in angstroms and bond angles are in degrees.
TABLE 111: Variation of Total Energya of Fluoroglyoxal with Dihedral Angle (Using STO-4G Basis Set) dihedral angle, deg
optimizedb
ab initio partially optimizedC
0 (trans) 30 60 90 120 150 180 (cis)
-323.33399 - 323.33257 -323.33178 -323.33298 -323.33480 - 323.33674 -323.33744
-323.34525 - 323.34363 -323.34081 -323.33974 -323.34111 - 323.34346 - 323.34460
MIND 0 / 3
MIND0/3-optimized trans Energies are in hartrees. geometry, treating the molecule as a rigid rotor. Structure partially optimized by an ab initio procedure with an STO-4G basis set (LCCF = 110", LOCF = 125"; all other parameters as in MIND0/3-optimized trans structure). The molecule is treated as a rigid rotor. a
occupied molecular orbitals for the trans and the cis conformers of chloroglyoxal.
Discussion The optimized geometry obtained for fluoroglyoxal in the trans conformation by using the MIND0/3 procedure is not unreasonable when compared with the known geometries of formyl fluoride17and methylglyoxal,ll with the exception of the CCF angle which seems somewhat small. However, when the dihedral angle is varied from 0" (trans) to 180" (cis) and the geometry is optimized at a series of fixed dihedral angles, all other geometrical parameters being optimized, an unexpected result is obtained (Table I). In particular the CCF angle decreases dramatically as the dihedral angle is increased along with a lesser but significant increase in the FCO angle. In the cis conformation the structure of fluoroglyoxalis highly distorted with a very short H--F distance of 1.243 A. The internal rotation potential shows two minima, one in the gauche
TABLE IV: Highest Occupied Molecular Orbitals in FluoroglyoxaP
MIND0/3
optimizedb
ab initio partially optimizedC
Trans -0.3543 n n(02Px,y FzPx,y' His) -0.4347 -0.4305 R - 0.3482
D-Z calculationd -0.4954 n -0.5896 n
R(RCO~ nCC*,CF)
- 0.4569
-0.4500 n
-0.6032 n
-0.4968
-0.6271
n( o z P x , y F'Px,y 1 9
- 0.4921
R
i~
n ( n C O , C C ~n C F * )
Cis -0.3514 n -0.4307 R -0.4553 n -0.4979 71
- 0.3499 n - 0.4341 n - 0.4602 n - 0.4919 n
a Energies are in hartrees. Using MINDO/S-optirnized trans geometry and an STO-4G basis set. Using partially optimized (ab initio) structure and an STO-4G basis set. Using MI~DO/3-optimizedtrans geometry and a (13S, 7P/4S, 2P) basis set.
TABLE V: Partially Optimized Geometry for Chlorogl yoxala Rc-Cl
Rc=o(Cl) RC-c
Rc=o(H) Rc-n
1.750 1.178 1.496 1.187 1.137
L OCCl L CCCl L L HCC
occ
128.3 110.0 129.9 108.9
a Bond lengths are in angstroms and bond angles are in degrees. Geometrical parameters are taken from fluoroglyoxal, except for optimization of LCCCl (using STO-4G basis set).
of Physical
Chemistry, Vol. 83, No. 21, 1979
J. Tyrrell, J. G. Ellison, and S. Stayton
TABLE VI: Variation of Total Energy of Chloroglyoxal with Dihedral Angle (Using STO-4G Basis Set)a
70 program and the STO-4G basis set as was the OCF angle which in MIND0/3 optimization came out a little larger than expected. The results of this optimization were to increase the CCF angle to 110' and to decrease the OCF angle to 125', bringing both angles into agreement with experimental data for comparable molecular ~ y s t e m s . ~These ~J~ optimized values for the CCF and OCF angles along with the remainder of the MINDO/a-optimized trans geometry were used to obtain a new set of total energies for various dihedral angles (Table 111). This set of results gives the trans conformer as being more stable than the cis conformer by 0.4 kcal/mol. The trans-cis energy difference corresponds in this case to having a mixture of trans and cis conformers, with the latter being present in about 33% of the total at room temperature. By analogy with the results obtained for oxalyl fluoride5with the STO-4G and the 4-31G basis sets, use of the smaller basis set underestimates the energy difference between the conformers and therefore overestimates the percentage of the cis conformer present. Unfortunately, attempts to use the 4-31G basis set in calculations on fluoro- and chloroglyoxal resulted in nonconvergence. The barrier to internal rotation was 3.46 kcal/mol relative to the trans conformer with a maximum in the potential curve at 90'. When the Fourier series expansion 2V(4) = C,V,(l- cos n4) is used to represent the internal rotation potential, where 9 is the dihedral angle, a two-term expansion gives values of Vl and V2 of 0.23 and 3.64 kcal/mol, respectively. If a three-term expansion is used, then the respective values of Vl, V,, and V3 are 0.12, 3.18, and 0.43 kcal/mol. In either case it is clear that the dominant contribution is from the V, term, emphasizing the stability of the planar, conjugated structures over the nonplanar structures. Steric hindrance effects appear to play a relatively small part in determining the nature of the potential curve. The rnolecular orbital energies and orbital types for the four highest occupied orbitals for both the trans and cis conformers of fluoroglyoxal are shown in Table IV. The order is n, T , n, T , in terms of increasing Koopman's theorem ionization potential, for both trans and cis conformers for both geometries used, and the orbitals are well separated in energy. Table IV also shows the orbital ordering obtained from a double-c basis set calculation carried out on the MINDo/s-optimized trans geometry of fluoroglyoxal. This larger basis set calculation gave a total energy for the trans conformer of -325.35172 hartrees but produced no change in the order of the highest occupied orbitals. The double-f basis set calculation took over 3 h of CPU time on our IBM 370/158 computer, so it was not considered to be practicable for extensive use on fluoroglyoxal. It was not possible to carry out calculations on chloroglyoxal by using the MINDO/3 program since it was not parametrized for chlorine-oxygen interactions, so all calculations on chloroglyoxal were carried out with the GAUSSIAN 70 program and an STO-4G basis set. Table VI shows the total energies as a function of the dihedral angle and indicates minima in the internal rotation potential for the trans and cis dihedral angles and a trans-cis energy difference of 1.93 kcal/mol in favor of the trans conformer. This energy separation would indicate that the cis conformer should only be present to the extent of 3.6% at room temperature. The barrier to internal rotation is 4.14 kcal/mol relative to the trans conformer. When a truncated Fourier expansion is used to represent the internal rotation potential as before, a two-term expansion gives values for V1 and V2 of 1.34 and 4.15 kcal/mol, respectively, while a three-term expansion gives values for V1,
2784
a
The Journal
dihedral angle, deg
total energy, dihedral hartrees angle, deg
total energy, hartrees
0 (trans) 30 60 90
-681.76306 -681.76101 -681.75757 -681.75647
-681.75777 - 681.75942 -681.75999
120 150 180 (cis)
The molecule is treated as rigid rotor.
TABLE VII: Highest Occupied Molecular Orbitals in Chlorogly oxal" trans -0.3605 n(02py, C13px,y) -0.4128 n(Cl,pz) -0.4230 n(C13px,y) - 0.4 6 18 n ( OzPx, ) -0.4816 n ( n c 0 , nee*)
cis -0.3592 -0.4181 -0.4307 -0.4611 -0.4832
n(02px y , %pY) n(C1,pZ',Ozpz) n(Cl,px,y) n(OlPxy) n(nc~,'ncc*)
Energies are in hartrees.
conformation (dihedral angle 120') and the other in the cis conformation, with almost identical energies and at a much lower energy than the trans structure. If, however, these calculations are repeated with the additional constraint that the CCF angle is held fixed at its value in the optimized trans structure, the variation in geometry and total energy with dihedral angle is much more reasonable (Table 11). When results listed in Tables I and I1 are compared, it is clear that the fluoroglyoxal geometries have already begun to diverge at a dihedral angle of 30°, but most of the divergence seems to occur between dihedral angles of 30 and 60'. This corresponds with an H.9-F distance of around 3 A. However, no such distortion is observed in the geometry in which the CCF angle is held fixed, even at H-.F distances less than 3 A. It would appear that, in the absence of any constraints on the geometrical parameters controlling the H.-F separaton, the parameterization of the H.-F interaction in MINDO/~is such that at H-F distances of around 3 A or less the effect is to maximize H-F bonding with corresponding distortion of the molecular structure. It should be mentioned that a similar effect was observed by us in using the MIND0/3 program to study the effect on the geometry of fluoroform (HCFJ of gradually lengthening the H-C bond. The internal rotation potential for fluoroglyoxal as expressed by the total energies in Table I1 shows a minimum at a dihedral angle of 90° and maxima at the trans and cis conformations. This is not in line with the expectation of a planar trans structure being dominant but is not unusal for internal rotation calculations. using the CNDO or MIND0 levels of approximation. For instance, both glyoxal, using a CNDO method,18 and biacetyl, using MIND0/3, show minima in the internal rotation potential only for a nonplanar structure. When the MINDo/3-optimized trans geometry of fluoroglyoxal is used with the GAUSSIAN 70 ab initio program and an STO-4G basis set, treating the molecule as a rigid rotor, a more typical internal rotation potential curve is obtained (Table 111). This potential has minima for both the trans and cis structures but with the unexpected result that the cis conformer appears to be more stable than the trans conformer by 2.16 kcal/mol. This is in contrast to the fact that in all simple 1,2-dicarbonyls previously studied, the trans conformer is the dominant species. It was previously indicated that the CCF angle obtained in the MIND0/3 optimization procedure was small relative to the CCF angle determined for comparable molecules. As a result this angle was reoptimized by using the GAUSSIAN
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The Journal of Physical Chemistry, Vol. 83, No. 21, 1979
MO Calculations for Glycine Crystals
V,, and V3of 1.15, 3.13, and 0.85 kcal/mol, respectively. As with fluoroglyoxal, the Vzterm is the dominant one but the V1 term is significantly larger, suggesting a greater degree of steric hindrance in the cis conformer of chloroglyoxal than in the trans conformer. The ordering of the highest occupied molecular orbitals in chloroglyoxal (Table VII) is significantly different from that in fluoroglyoxal (Table IV). As with fluoroglyoxal, the highest occupied orbital is a nonbonding, in-plane orbital associated primarily with the oxygen but having significant halogen p character and hydrogen 1s character. However, while the second highest occupied orbital in aCF), the fluoroglyoxal is clearly of T character (aC0,aCC*, equivalent orbital in chloroglyoxal is almost entirely chlorine 3p, and should be classified as nonbonding. The third highest occupied orbital, which is in fluoroglyoxal predominantly oxygen in-plane nonbonding, is in chloroglyoxal a chlorine 3p in-plane nonbonding orbital. The next two molecular orbitals of chloroglyoxalare, in order of increasing Koopman's theorem ionization potential, an oxygen, in-plane, nonbonding orbital and a T orbital essentially equivalent to the second highest orbital in fluoroglyoxal. These results would indicate that, unlike fluoroglyoxal, the oxalyl halide^,^ glyoxal,3 and biacety1,'O a* electronic there is little likelihood of finding a
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transitions among the n
2785
a* transitions of chloroglyoxal.
References and Notes (1) J. C. D. Brand, Trans. Faraday Soc., 50, 431 (1954). (2) G. N. Currie and D. A. Ramsay, Can. J . Phys., 49, 317 (1971); J. R. Durig, C. C. Tong, and Y. S. Li, J. Chem. Phys., 57, 4425 (1972). (3) C. E. Dykstra and H. F. Schaefer, J. Am. Chem. Soc., 97, 7210 (1975). (4) J. R. Durig, S. C. Brown, and S. E. Hannum, J. Chem. Phys., 54, 4428 (1971). (5) J. Tyrrell, J. Am. Chem. Soc., 98, 5456 (1976). (6) J. Goubeau and M. Adelhelm, Spectrochim. Acta, Part A , 28a, 2471 (1972). (7) J. R. Durig and S. E. Hannum, J. Chem. Phys., 52, 6089 (1970). (8) K. Hagen and K. Hedberg, J. Am. Chem. Soc., 95, 1003 (1973). (9) K. Hagen and K. Hedberg, J. Am. Chem. Soc., 95, 8266 (1973). (10) J. Tyrrell, J. Am. Chem. SOC., 101, 3766 (1979). (1 1) C. E. Dyllick-Brenzinger and A. Bauder, Chem. Phys., 30, 147 (1978). (12) R. R. Lucchese and H. F. Schaefer, J. Chem. Phys., 68, 769 (1978). (13) M. J. Dewar, H. Metiu, P. J. Student, A. Brown, R. C. Bingham, D. H. Lo, C. A. Ramsden, H. Kollmar, P. Weiner, and P. K. Bischof, ~ ~ 3 1general 3 , IBM version, modified by M. L. Olson and J. F. Chiang, QCPE No. 309, Quantum Chemistry Program Exchange, Indiana University, Bloomington, Ind. (14) W. J. Hehre, W. A. Lathon, R. Ditchfield, M. D. Newton, and J. A. Pople, GAUSSIAN 70, QCPE No. 236, Quantum Chemistry Program Exchange, Indlana University, Bloomington, Ind. (15) E. Clementi, J. Mehl, and H. Popkie, "IBMOL 5A User's Guide", IBM Research Laboratory, San Jose, Calif. (16) S. Huzinaga and Y. Sakai, J . Chem. Phys., 50, 1371 (1969). (17) R. F. Miller and R. F. Curl, J. Chem. Phys., 34, 1847 (1961). (18) 0. Gropen and H. M. Seip, Chem. Phys. Lett., 11, 445 (1971).
Molecular Orbital Calculations for Glycine Crystals Z. Latajka and H. Ratalczak" Institute of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50-383 Wroclaw, Poland (Received January 29, 1979)
The Bacon-Santry perturbation method in CNDO approximation has been employed to study three-dimensional glycine crystals in a , 0, and y forms. It is found that the a-glycine crystal is more stable by about of 6.9 kcal mol-' than the P-glycine crystal and about 8.8 kcal mol-l more stable than the y-glycine crystal. The change of molecular properties in the gas phase monomer and in crystals has been discussed.
Introduction As a simple a-amino acid, glycine has the priviledged role of a model compound in quantum biochemistry. As the very simplest molecule which will form a peptide bond, it is of great interest in terms of understanding the electronic and conformational structure of proteins. Glycine is known to exist in a variety of forms, depending on its environment. In the isolated state it exists in the canonical form, H,NCH2COOH.1-3 In solution, the glycine molecule is well known to exist as the zwitterion, H3N+CH2COO-,at biological pH, and as glycinium ion, H3N+CHzCOOH,at low pH, while in the solid state it takes on variety of structures. Glycine itself in the crystalline state exists in three polymorphic forms. The ordinary state, a, crystallizes readily by slow evaporation of neutral solution^.^^^ The P form crystallizes by adding ethyl alcohol to a concentrated aqueous solution of glycines6Crystals of the P form readily transform into the a form in moist air. The third form, y-glycine, crystallizes by slow cooling of aqueous solutions of glycine made acidic with acetic acid.' Moreover, the y form is also obtainable by appropriate treatment of the 6 form with water. The crystal of yglycine shows a marked piezoelectric property along the c axis.4 0022-365417912083-2785$01 .OO/O
Recently, Bacon and Santry8-lodeveloped a SCF perturbation theory for molecular crystals based on localized orbitals of the molecules in the unit cell. The CNDO version of this method was applied by Santry et al. to the studies of three-dimensional crystals of HF,1° ice,12-14and urea.16 The total energies of unit cells and the geometry of the molecules in the crystals were reproduced satisfactorily in these calculations. However, recently Santryl1J6 has shown that this method in the CNDO/2 approximation has a tendency to predict nonpolar lattice structures to be more stable than polar ones for simple hydrogen bonded crystals. In the present paper we have reported the studies of the three-dimensional glycine crystals in a , 0, and y forms by using the Bacon-Santry perturbation method in the CNDO approximation. Method of Calculation A general formalism of the Bacon-Santry perturbation method for the evaluation of molecular crystals has been given in the original paper,a1o so that we mention here only a few essential points. In this method the crystal energy per molecule was expressed as the sum of five terms: Wcrys = Wmol + Welec + Wpolarizn + Wintermol + Wintramol 0 1979 American Chemical Society
