5386
J. Phys. Chem. 1983, 87,5386-5388
heterobicyclo[ 3.2.11 octane systems. Cremer’s extended basis set ab initio calculations on the structure of ethylene ozonide over its pseudorotational potential surfacee”predict a lengthening of the O,-O bond by about 0.01 A when going from the C2 to the symmetry conformer. The structural parameters of the C2 and C, symmetry conformers of ethylene ozonide calculated by Cremer are shown in Table VI1 along with some experimentally determined r, structural parameters of ethylene and cyclopentene ozonides. It is interesting that a lengthening of the 0 -0 bond by about 0.01 is observed in CpOz as well 89 t i e sfight lengthening of the C-0, and shortening of the C-0, bond lengths that were predicted. These observations support Cremer’s interpretation that ethylene ozonide is forced into the Cz conformer not only by dipole-dipole repulsions but also by the delocalization of the oxygen lone-pair electrons in the C-0 Q* orbital in the twisted conformer. However, it may be unwise to place too much emphasis on the comparison between ethylene ozonide and CpOz since the bond changes are small and the additional ring constraints in CpOz complicate comparisons. This is probably the reason why the trends in
6
predicted and observed bond angles, particularly dihedral angles, are quantitatively more disparate. It is also noteworthy that the predicted plane angle 4 (Figure 1)is 136.7’ in CpOz but a flatter envelope (145.6’) is predicted for C, ethylene ozonide.
Acknowledgment. This study was supported by Grant CHE8005471 from the National Science Foundation, Washington, DC. We are grateful to Dr. Kurt Hillig for advice on many aspects of this study. Registry No. CpOz, 280-21-7; 180,-CpOz, 87801-46-5; l80,-
CPOZ,87801-47-6; 180280 -CpOz, 87801-48-7; 180~s0,-CpO~, 87801-49-8; 180~80~80,-Cp6z, 87801-50-1; 13C&pOz, 87801-51-2; 13C,-CpOz, 87801-52-3; 13CrCpOz, 87801-53-4; 2H,,eq-CpOz, 87801-55-6; 2H,,ax-CpOz,87801-56-7; 2Hf,eq-CpOz, 87801-57-8; 2Hf,ax-CpOz,87801-58-9.
Supplementary Material Available: Tables Sl-Sl8 listing the transition frequencies for the 14 isotopic species, the three excited vibrational states of the normal species, and the ground-state rotational constants of the substituted isotopic species (22 pages). Ordering information is given on any current masthead page.
Energy Component Analysis Calculations on Interactions Involving I, and H I U. Chandra Singh and Peter Kollman” School of Pharmacy, Department of Pharmaceutical Chemistry, University of California, San Francisco, California 94 143 (Received: February 16, 1983; In Final Form: April 28, 1983)
We present ab initio calculations on the complexes H2CO-HI, H2C0.-IH, H2CO-.I-CH3, H3N.-12, H3N-IH, H3N-.HI, and CH3NH2-12. Morokuma component analyses of these complexes have been carried out in order to elucidate the nature of these interactions. In particular, it is of interest to compare nitrogen and oxygen base interactions with iodine, to compare the Lewis acid properties on each “endnof H-I, and to examine the methyl substituent effect in amine-4, interactions. For HzCO--IH, we also carry out estimates of the correlation energy (MP2 level) contribution to the intermolecular attraction.
Introduction It is clear that ab initio electronic structure methods are very powerful tools for understanding the strength and directionality of intermolecular interactions. However, they still have been applied mainly to molecules containing atoms of only the first two rows of the periodic table. Although pseudopotential methods should ultimately allow routine applications to molecules containing heavier elements, it is still somewhat of a challenge to study iodinecontaining molecules. This is unfortunate, since iodinecontaining molecules often have unusual spectral properties and this atom also plays an important biological role in thyroxine and its analogues.2 Thus, a more precise understanding of the nature of alkyl and aryl iodine interactions with other molecules is of importance. We have previously carried out quantum mechanical calculations on the interactions of H3N with Iz and HI, using ab initio SCF t h e ~ r y . We ~ now extend this study ~~~
~~
in several important ways. First, we examine the methyl substituent effect on iodine interactions, by comparing the interaction of CH3NH2with 1, with that of NH, with 12. In an earlier study of alkyl-amine interactions with SO2, we have found4 a very large methyl substituent effect on this interaction and we wish to see whether this is also found in amine-I2 interactions. Secondly, we wish to use Morokuma energy component a n a l y ~ i sboth , ~ to further understand the NH,-IH, NH3.-HI, and NH3-.12 systems, which were subjects of the previous study, and to see how such components are effected on changing the Lewis base from NH, to CH3NH2. Finally, we wish to examine the structure and energy of R-14=CH2 complexes, as well as the energy components for such interactions, because the nature of I-.O interactions observed in the solid state have been characterized in terms of R-1-0 distance and R-1-0 angle.6 In our earlier study,l we merely inferred the likely results of 1.-0 ab initio studies from the 1.-N results; here we carry out
~
(1)R. Mulliken, J. Am. Chem. SOC.,74,811 (1952). (2)For an analysis of the structure activity relationships of thyroxine analogues, see S. Dietrich, M. Bolger, P. Kollman, and E. C. Jorgensen, J.Med. Chem., 20,863 (1977). (3)P. Kollman, A. Dearing, and E. Kochanski, J. Phys. Chem., 86, 1607 (1982). 0022-3654/83/2087-5386$01.50/0
(4)J. Douglas and P. Kollman, J. Am. Chem. SOC., 100, 5226 (1978). (5)K. Kitaura and K. Morokuma, Znt. J . Quant. Chem., 10, 325 (1976). (6)P.Murray-Rust and W. Motherwell,J.Am. Chem. SOC., 101,4374 (1979).
0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 26, 1983
Energy Component Analysis Calculations on I, and H I
5307
TABLE I: SCF Interaction Energies, Energy Components, and Geometries of Complex Involving HI, I,, and H F complex
B...A CH ,0... H I CH,O*.,HF CH,O..,IH CH,O..,ICH, CH,HCO...IH H,N,..HI H,N...HF H,N...IH H,N,.,I, CH,H,N,..I,
Ra 3.90 (2.68) 3.37 3.34 3.37 3.40 2.67 3.20 2.67 2.26
0
47 (40y 47 (47)' (47)' 0 0 0 0 0
-AEESCq
-AEpold
- A E c T ~ + ~ ~- A E c T B + A ~
A Emlxg
A
E
3.07 13.03 1.34 0.79
0.42 1.49 0.19 0.23
0.61 0.29 0.54 0.56
1.45 2.63 0.76 0.80
0.14 -0.38 0.03 -0.06
2.98 7.84 1.25 1.56
21.90 25.39 4.83 25.18 29.34
2.20 1.94 0.61 2.70 3.17
4.67 0.52 0.78 3.63 4.74
7.53 3.52 1.59 8.95 10.56
0.72 0.70 -0.04 0.08 0.38
27.84 15.80 4.67 30.10 36.98
~ -AE~ t o t i~
2.43 9.99 1.16 0.87 1.70 7.73 16.26 3.18 10.28 11.22
Optimized angle in degrees; values in parentheses if angle assumed a Optimized distance in A between N or 0 and F or I. Polarization energy (kcalimol). e A + B Electrostatic interaction energy (kcal/mol). or taken from another study. charge transfer energv fkcal/molL f B + A charge transfer energy (kcalimol). g Component mixing energy (kcalimol). e assumed same as for H,CO...IH . Exchange repulsion (kcal/rnol): Total energy( kcalimol). e,R from ref 12. J
direct calculations on the 1-0 interactions at both the SCF and MP2 levels. Methods We used the program Gaussian 80 UCSF in these calcul a t i o n ~ . The ~ program was run on the Structural Biology DEC VAX 11/780 a t UCSF. We used a 4-31G basis set' for H, N, C, 0, and F and the valence-shell double l basis set for I developed by Kochanski? The energy component analyses were carried out as described by Morokuma et al.5 The monomer geometries of CH20,CH,CHO, HI, HF, CH,I, NH,, and CH3NH2were taken from experiment and the 1-1 distance of Iz from previous calculations3 and the total energies for these molecules are listed in footnote 10. Correlation energy calculations were carried out a t the MP2 1evel.ll Results and Discussion Table I contains the results of studies on CH20-.HI, CH,O.-HF, CH,O.-IH, CH20-.ICH3, and CH,CHO-IH interactions. In each case we assumed that the O--XY angle was 180°, and optimized the geometry with respect to the CO.-X angle and 0-X distance. Also, in Table I we present the results on H,N-HI, H,N.-HF, H,N-.IH, H3N...Iz, and CH3NHz.-12interactions. In these cases, we have previously optimized only the Ne-X distance, assuming the approach of X along the C3 axis. We have previously presented3 the results of geometry optimization of H3N.-IH, H,N-HI, and NH3-.12 complexes, in which in the latter two cases we also optimized the H-I (1-1) intramolecular distances. But here we present only the N...X optimized results for these systems and add to this the N-I optimization results of CH3NH2-I, with Morokuma component analyses on all four complexes. We first discuss the results on the hydrogen bond complexes CH,O-HI, CHzO-.HF, H3N-HI, and H3N--HF. Not surprisingly, the stronger interaction of HI occurs with H3N. What is somewhat surprising is the unusually large difference in distance between these two complexes. This is in contrast to the results for various first row proton donors H-F, H-OH, and H-NH2, where the distances of (7) Quantum Chemistry Program Exchange Program No. 446, QCPE Bull., 2, 17 (1982). (8) R. Ditchfield, W. Hehre, and J. Pople, J. Chem. Phys., 54, 724 (1971). (9) J. Prisette, G. Seger, and E. Kochanski, J. Am. Chem. SOC.,100, 6941 (1978). The iodine basis set is 15s, llp, 7d contracted to 7s,5p, 3d. (10) The total energies (in atomic units) of the monomers were (at the SCF level) as follows: HF, -99.887 26; H,CO, -113.69261; CH,HCO, -152.685 86; H3N,-56.10333; CH3NH2,-95.06978; HI, -6909.807 26; CH31, -6948.79653; and I*,-13 818.473 11. At the MP2 level, the energies were CH,O, -113.910 74, and HI, -6 909.924 53. (11) J. S. Binkely and J. Pople, Int. J . Quant. Chem., 9, 229 (1975).
a given donor to the acceptors FH, OHz,and NH3 is almost identical.12 Also, in interactions of Li+ with O=C and NH3, the Li-0 distances is shorter than the Li.-N distance, as one would expect from the relative van der Waals radii of the atoms.13 This significant difference between these Li+-.base and I-H-base interactions comes about because of the much greater contribution of charge transfer in the base-HI interactions. In the case of Li+-base interactions, the sum of charge transfer and mixing energies is actually >0, whereas in the I-H interactions studied here the sum of the charge transfer and mixing energies are a significant fraction of the attractive interaction energy. Morokuma component analysis calculations with the 4-31G basis set have been carried out on H3N-HF and H,CO.-HF (Table I). Thus, we can compare H3N-.HX and H,CO-HX for X = F and X = I. The differences are significant: the first row X = F H bonds are dominated by the electrostatic energy,14which is 3-4 times the sum of the other attractive components. In the case of X = I, the electrostatic term is only 1.4-1.6 times the sum of the other components. This simply related to the fact that I has a greater polarizability and greater stability of its H-X antibonding u* orbital than F. Elsewhere, we have contrasted4 the R3N--Li+and R3N--SOzinteractions and have noted that, for interactions that are electrostatic dominated (Li+-NR3), the N-X distance is relatively independent of R. When the charge transfer term is more important (SO2-NR,), the S--N distance is quite sensitive even to methyl substitution on the amine. Here we see a related effect; the 0.-H-F and N--HF minimum energy distances are similar, because the interaction is electrostatic dominated, but the 0.-H-I and N.-H-I distances are dramatically different, because the interaction contains a significant contribution from charge transfer and polarization. One of the most interesting results of our previous study of N3H-HI and H3N--IH was that both were attractive interactions, in contrast to the corresponding interactions involving HF, where H3N-.HF is very attractive but H3N--FH completely repulsive. This is clearly due to the fact that, for H-F, its dipole moment (F(ca1cd) = 2.28 D) is larger in magnitude compared to its quadrupole moment (O(calcd) = 1.72 B; B = Buckinghams = lo-%esu cm2)(and is more important at the equilibrium dimer distances) and the NH3-.FH interactions is electrostatically repulsive. On the other hand for H-I, its quadrupole (O(calcd)= 2.96 B) (12) P. Kollman, J. Mc Kelvey, A. Johansson, and S. Rothenberg, J . Am. Chern. SOC.,97, 955 (1975). (13) P. Kollman and S. Rothenberg, J . Am. Chern. SOC.,99, 1333 (1977). (14) H. Umeyama and K. Morokuma, J . Am. Chern. SOC.,99, 1316 (1977).
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The Journal of Physical Chemistty, Vol. 87, No. 26, 1983 0 6 +1-16+ 6-
is quite large compared to its dipole moment (p(ca1cd) = 1.01 D) and, as noted in Table I, the electrostatic component for H,N-IH is very attractive. For the corresponding halogen interaction NH3...12 the Lewis acid has no dipole moment so only the substantial quadrupole moment ” -
6 +H&
+
6-
makes this interaction very attractive electrostatically, although at this short distance the exchange repulsion is even greater in magnitude. It is interesting though that, in both H,N-IH and H,N.-I,, the ratio of the electrostatic to the charge transfer and polarization and mixing energies are -1.6, very similar to the H-bonded interaction H3N-H-I. Not surprinsingly, the CH,O-IH interaction is substantially weaker than H,N.-IH, as was found in comparing H3N-HI and CH,O-HI. Interesting, this weakest interaction CH20-IH has the smallest ratio of electrostatic/ polarization and charge transfer and mixing energies, but the C=O--I angle is almost the same as C=O--H. Finally, we focus on the methyl,substituent effects. We studied the CH,O.-I-CH,, interaction because much of the X-ray data on C=O--I interactions involves alkyl (or aryl) iodides. Interestingly, this interaction is actually slightly weaker than CH,O-.IH, although the distance is somewhat shorter. Examining the energy components makes clear why this is so. The electrostatic energy is less favorable for CH20-.I-H than for CH20-I-CH3, but the charge redistribution effects cause the 0.-I distance to be slightly shorter. This is rather unexpected, in that methyl substitution weakens the interaction, but shortens the 0-1 bond. We can understand this from the fact that substituting a CH, for H increases the dipole moment of RI, thus destabilizing the complex, but this seems to come about by I CH, u charge transfer, somewhat balanced by CH3 I T charge transfer. It is the reduction in the u population on iodine in CH31 compared to HI which causes the lpwer exchange repulsion for this complex. Interestingly, the interaction energy and distance of CH,HCO--IH are essentially identical with those of H2CO--IH, despite the fact that CH,HCO has a larger dipole moment than H2C0.13 Such an effect has precedence, in fact, in the studies on the methyl substituent effect on H bonding by Tse etf al.15 They found a small weakening and shortening of the RO-He-0 hydrogea bond when R was changed from H to CH3. We next turn to the methyl substituent effect in H,N-12. Earlier, we studied this effeot in H3N.-S02 and concluded that the charge transfer term was responsible for the significantly shorter (-0.2 A) N-S distance and 3 kcal/mol stronger interaction in CH3NH2.-SO2than H3N-.S02. In the case of CH3NH2-.12vs. H3N-.12, we do not calculate as large of a methyl substituent effect, with a shortening of the N-.I distance of only 0.05 A and an increase in the -AE of 1kcal/mol. For RR’R’’N.-SO2, we studied the singly, doubly, and triply CH3 substituted amine, but by far the most substantial shortening of the N-S distance and strengthening of the interaction occurred with the addition of the first methyl group. Thus, it is surprising
--
-
(15) M. Newton and N. Kester, Chem. Phys. Lett., 94, 198 (1983).
Singh and Kollman
that the methyl effect in R3N.-12 interactions is so small. We have suggestive evidence that the effect of correlation energy is to reduce the distance in these 1.-X complexes. We studied the effect of including the correlation energy contribution at the second order Moller Plesset (MP2) level on the complex CH,O-IH, keeping the CO.-I angle fixed at the 47O found at the SCF level. The minimum energy 1-0 distance was 3.37 8, at the SCF level; it was significantly reduced to 3.10 A at the MP2 level. The -AI3 for complex formation increased from the 1.61 kcal/mol found at the SCF level to 2.55 kcal/mol at the MP2 level. Even at an I--O distance of 2.9 A, among the shortest found in the survey of crystal structures,Gthe -AE was still significantly attractive (1.85 kcal/mol). Improvements in the basis set would tend to increase the O--I distance via the smaller basis set superposition error, whereas more precise correlation energy estimates might decrease the distance. Adding methyl groups to CH20.I-H would be likely to (slightly) shorten the 1-0 distance through dispersion effects and it is not clear what the effect of crystal forces would be. However, all in all, the qualitative agreement between the potential surface we have found for H,CO.-IH and (in the previous paper) H3N-.I-H and H3N-.IH should be considered satisfactory, particularly in view of the recent studies by Newton and Kestner,16in which the difficulty of estimating the counterpoise correction even in very accurate calculations on (H,O), was emphasized. It is likely that correlation effects will also contribute to alkyl amine-I, interactions, and if we assume that adding two methyl groups to CH3NH2-.12to make (CH,),N--I, will decrease the N-I distance by -0.1 A and correlation effects will decrease this distance further by the same amount as in R2C0.-IH (0.27 A), we would estimate an N-I distance of 2.25 A, in reasonable proximity to the observed crystal structure1, value of 2.27 A. Of course, the above caveats on the effect of limited basis set, limited correlation energy, and crystal forces also are of relevance, in addition to the need to allow the 1-1 distance to “relax” upon optimizing the N--I distance.
Summary and Conclusions We have used Morokuma component analyses to compare and contrast the nature of base-HF and base-HI, and base.-IH interactions. Not surprinsingly, charge transfer is more important in the iodine interactions but is certainly not “dominant”. We have included correlation effects by calculating the energies for H,CO-IH at the MP2 level; such calculations allow us to qualitatively rationalize the 0-1 distances observed in crystals which are well below the sum of the 1-0 van der Waals radii. Such correlation effects are likely to be important in calculations of amine-I, complexes and, if one assumes that they are, our estimate for the N-I distance in (CH,),N-I, is close to the experimental value. Acknowledgment. We are pleased to acknowledge the support of the NSF (CHE-80-26560)in this research. We are also grateful to E. Kochanski for sending us her iodine basis set. Registry No. Formaldehyde, 50-00-0; hydrogen iodide, 10034-85-2; iodomethane, 74-88-4; ammonia, 7664-41-7; iodine, 7553-56-2. (16)Y.C.Tse, M. Newton, and L.C. Allen, Chem. Phys. Lett., 75,350 (1980).
