10280
J. Phys. Chem. 1991, 95. 10280-10284
we obtain using eqs A1 and A2 a three-dimensional surface of the type E = E(rl,r2,R)depending on the parameters Ro, EAo, Em, EEO,and EEC.Equation Al-A3 are valid only for the case where EE > EB. The reaction energy along the stepwise pathway ( E = E(ras)) is given by - EA
+ ( E E _R2-_ E B ) (rash + 2 1 / 2 R )+2
E = E E - p b2ra,2 R2 3(2EE - E, - EB)
The minimum of E*,,(R) is located at + + EE - EB (rash + 2 1 / 2 R ) 4 , 4R4 b = [{I + [(EE- EB)/(EE - E ~ ) ] l ~ ~ )(A4) l~~]-l
4R4
EA
b2raS2(ra,b 2 1 / 2 R ) 2 'as4 + 4R4
For a given R value the barrier of the stepwise motion is located at ,Js
R ( ~ E E-oEAO- ~ E B o ) = -2EEO-EAO-EB0 '
For the energy E*,,(R) of the saddle point along the stepwise pathway we obtain
E!,(R,) is the true barrier for the stepwise motion. The corresponding value for the concerted proton motion E*,(R,) is obtained by setting a = E E C / ( ~ ( E-EEOA ) ) 648) in eq A7. Registry No. DFFA, 18464-36-3; DZ,7782-39-0. Supplementary Material Available: Tables 1-6 containing all numerical data obtained by data analysis of the N M R experiments described in this study (7 pages). Ordering information is given on any current masthead page.
Deprotonation Energy and Charge Redistribution in Excited States of Acetylene? Vijaya Marudarajan and Steve Scheiner* Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901 (Received: March 8, 1991)
UHF and UMP3 level calculationswith a 6-31+G** basis set are used to investigate the ground and first few excited electronic states of HCCH and its deprotonated anion, HCC-. All excited states are found to adopt a bent geometry, some cis and some trans in the case of HCCH. The deprotonation energies of these excited states do not differ much from that of the ground state. The bending of the molecule leads to a transfer of electron density from C to H when an electron is deposited into the lower A* virtual MO. This trend is surprisingly invariant with respect to the particular excited state.
Introduction The transfer of a proton from one group to another underlies all of acid/base chemistry.'V2 Within the regime of biology, such transfers are vital to the functioning of a number of Photon-induced proton transfers have been shown to be a viable means of inducing lasing activity.610 The first step in the latter process involves the electronic excitation of a molecule containing a hydrogen bond. The properties of the H-bonding groups are altered within the excited state such that a proton transfer occurs rapidly and exoergically. The subsequent relaxation to the ground state sets the stage for the reverse transfer of the proton back to the original configuration. A large body of work has accumulated over the years concerning hydrogen bonding and proton transfers within the ground electronic state.'2."J2 Recent ab initio computation^'^-^^ have outlined the importance of such features as the relative proton affinities of the two groups, their distance of separation, and the angular aspects of their relative geometry. In comparison, very little is clearly understood about the parallel transfer in the excited state. In any case, it is certainly reasonable to expect the proton affinities 'Dedicated to Professor Michael Kasha on the occasion of his 70th birthday.
0022-3654/91/2095-10280$02.50/0
of the various groups in their excited states to play a crucial role in proton transfers between them in the corresponding state. For (1) Bartmess, J. E.; McIver, R. T., Jr. In Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic Press: New York, 1979; Vol. 2, Chapter 11. (2) Caldin, E. F.; Gold, V. Proton-Transfer Reactions; Wiley: New York, 1975. (3) Silverman, D. N.; Lindskog, S. Acc. Chem. Res. 1988, 21, 30. (4) Cho, Y.-K.; Cook, P. F. Biochemistry, 1989, 28, 4155. (5) Craik, C. S.;Roczniak, S.; Largman, C.; Rutter, W. J. Science, 1987,
237, 909. (6) Parthenopoulos, D. A.; Kasha, M. Chem. Phys. Lett. 1988, 146, 77. (7) Kasha, M. Acta Phys. Pol. 1987, A71, 717. (8) Chou, P.; McMorrow, D.; Aartsma, T. J.; Kasha, M. J . Phys. Chem. 1984, 88, 4596. (9) Barbara, P. F.; Walsh, P. K.; Brus, L. E. J . Phys. Chem. 1989,93, 29. (IO) Itoh, M.; Hasegawa, K.; Fujiwara, Y. J . Am. Chem. SOC.1986, 108, 5853.
( I I ) The Hydrogen Bond. Recent Developments in Theory and Experiments; Schuster, P., Zundel, G., Sandorfy, C., Eds.; North-Holland: Amsterdam, 1976. ( I 2 ) Hydrogen Bonds; Boschke, F. L., Ed.; Springer-Verlag: Berlin, 1984. (13) Bosch, E.; Lluch, J. M.; Bertran, J. J . A m . Chem. SOC.1990, 112, 3868. (14) Cao, H. 2.;Allavena, M.; Tapia, 0.;Evleth, E. M. J. Phys. Chem. 1985, 89, 158 1. (15) Brciz, A.; Karpfen, A.; Lischka, H.; Schuster, P. Chem. Phys. 1984, 89, 337.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 25, 1991 10281
Excited States of Acetylene ..................
bE
.,..
w
BC,,.................
a2
_..I
N.".'.''w .............
b2
,501 a,
bu
-@L@(
............. 1:: __..' ._..
Hi-i.::::;:::
........
........
a1
w
"0-0, trans C2h
q-p
=U
linear D..h HCCH iT*
...
H#iT
bi
cis ClV
- - ................
................ ..__
.........
loot
@# a"
w
Energy, kcal/mol
5
d
a'
trans C2h
a"
w
HCCH
a'
CCH' Figure 1. Schematic diagram showing splitting of bonding and antibonding R orbitals of H C C H and HCC- as each moiety is bent.
this reason, it would be useful to have at hand some principles as to how electronic excitation affects the proton affinity of a given group. Another feature of the group which has a strong bearing upon its proton-transfer properties is its internal charge distribution. For this reason, another focus of research into the effect of electronic excitation concerns the accompanying charge rearrangements. The yne group of a molecule like acetylene engages in proton transfers in a very interesting manner, behaving in some ways like a "normal" proton donator, Le., like an electronegative 0 or N acid.2wz2 However, its properties are also somewhat similar to the carbon acids that release their proton slowly if at all. For this reason, proton transfers involving the triple bond have been investigated previously.20 However, the earlier work was devoted entirely to the ground electronic state. The current paper is in its lower devoted to investigation of the properties of H-H lying excited states. Specifically, we compute the change in the energy required to remove a proton and in the electronic density. Another intriguing aspect of this molecule is the difference in shape between its linear ground state and its planar bent excited states. Some inferences are drawn as to how these changes relate to its activity in a proton transfer.
Figure 2. Ordering of states in trans, linear, and cis H C C H a t the UMP3 level. State labels in boldface are the most stable. T h e energy scale applies to all curves shown except the uppermost 'Au state which was not calculated here.
Energy, kcal/mol 100
I
50c
linear
Methods The electron configuration of the closed-shell ground state of HCCH is ( 1 ( i o u~j 2 (2cg)2 (20,)2 (3a )2 Its low-lying excited states are known to be of zu T! valence The
-
cis Czv
linear D..h
bent
HCCFigure 3. Ordering of electronic states in linear and bent HCC- computed a t the correlated level.
configuration resulting from such a single (rJ37r* I excitation and can give rise to six Franck-Condon states: viz. 1*32'u, An earlier ab initio CID calculation by D e m ~ u l i nfound ~~ the energy ordering to be as follows: Ig3Z-,,,
(16)Scheiner, S . Acc. Chem. Res. 1985,18, 174. (17)Cybulski, S . M.;Scheiner, S . J. Phys. Chem. 1989,93, 6565;Ibid. 1990,94, 6106. (18)Scheiner, S.;Redfern, P. J. Phys. Chem. 1986,90, 2969. (19)Hillenbrand. E.A.: Scheiner. S . J . Am. Chem. Soc. 1986.108.7178. (20)Cybulski,S: M.;Scheiner, S.J . Am. Chem. SOC.1987,109, 4199. (21)Fraser, G. T.;Lovas, F. J.; Suenram, R. D.; Nelson, D. D., Jr.; Klemperer, W. J. Chem. Phys. 1986,84, 5983. (22)Bednar, R. A.; Jencks, W. P. J . Am. Chem. SOC.1985,107,7117, 7126,7135. (23)King, G.W.; Ingold, C.K. Nature 1952,169, 1101. (24)Ingold, C.K.; King, G.W. J. Chem. SOC.1953,2702. (25)Innes, K. K.J . Chem. Phys. 1954, 22, 863. (26)Trajmar, S.;Rice, J. K.;Wei, P. S.P.;Kuppermann, A. Chem. Phys. Lett. 1968,I , 703. (27)Lassettre, E.N.; Skerbele, A.; Dillon, M. A,; Ross, K. J. J . Chem. Phys. 1968, 48,5066.
IZ+*< 3Z+u< 3Au < ' s 3 2 - , < lAU < '2+,, (28) Dance, D. F.; Walker, I. C. Chem. Phys. Lett. 1973,18, 601. (29)Foo, P. D.; Innes, K. K. Chem. Phys. Lett. 1973,22, 439. (30) Van Veen, E.H.; Plantenga, F. L.; Chem. Phys. Lett. 1976,38,493. (31)Wendt, 13. R.;Hippler, H.; Hunziker, H. E. J . Chem. Phys. 1979, 70,4044. (32)Demoulin, D. Chem. Phys. 1975,I!, 329. (33) Wetmore, R.W.; Schaefer, H. F., 111. J . Chem. Phys. 1978,69,1648. (34)So,S . P.; Wetmore, R. W.;Schaefer, H. F., 111. J. Chem. Phys. 1980, 73, 5706. (35) Martin, P. S.;Yates, K.; Csizmadia, 1. G. J. Mol. Struct. (Theochem.) 1988,165, 353.
10282 The Journal of Physical Chemistry, Vol. 95, No. 25, 1991
Marudarajan and Scheiner
TABLE I: Geometries (Distances, A; Angles, deg) and Energies (Hartrees) of Ground and Excited States of Acetylene and HCCgeometry
state
Pt gP
r(CC)
r(CH) HCCH 1.057 1.079 1.074 1.080 1.093 1.078 1.086 1.085 1.083
3AIf
c,
1.189 1.318 1.319 1.382
trans
3AIt
c,
1.372
cis trans
]A2 'A"
c,, c2 h
1.332 1.361
linear
'E+
bent bent bent
'A' 'AI!
c-, c, c* cs
1.233 1.365 1.406 1.404
linear cis trans cis
h
I&+
Dm
'B2
C2r
3B"
'A"
C2h
B(CCH)
EUHF
E,IMP,
180.0 128.9 131.2 138.4 114.1 128.0 116.8 133.3 123.4
-76.827 22 -76.728 67 -76.71691 -76.691 29
-77.09683 -76.963 34 -76.950 33 -76.91881
-76.699 63
-76.935 40
-76.672 89 -76.686 57
-76.909 46 -76.924 39
180.0 127.3 112.8 113.1
-76.21244 -76.101 50 -76.090 10 -76.080 5 5
-76.479 48 -76.337 93 -76.324 39 -76.31398
HCC
However, the energies of the orbitals are lowered by bending of the molecule, as shown in Figure 1. The most significant change is undergone by the a* MO, which is greatly stabilized by the contribution of the hydrogen orbitals in either the trans or cis type of distortion. As a consequence, the lowest energy excited states are those which involve this MO, ag in the case of trans bending and b2 for cis. These excited states all adopt a bent equilibrium geometry. The lowest energy excited state is the 3E+u,which distorts to 3B, (trans) or 3B2(cis), as illustrated in Figure 2. The linear 3Austate splits into )A, and 3B, in the trans arrangement and into 3A2and 3B2for cis. In either case, the A symmetry results in a lower energy. The B states, higher in energy, are only weakly bent. Moreover, prior work has indicated added stability by dihedral twisting toward a gauche C2 structure.32 The excited singlet state of lowest energy is 'E-,,which bends into 'A, and IA2 for trans and cis, respectively. Still higher in energy is the ]A,, singlet, and its bent correlates. The situation for the HCC- anion is quite similar in that the lower energy excited states are all bent. While the ground state is linear I P , the higher energy linear 3Z+, 3A, and ]E-states resulting from excitation to the a* MO bend to 3Af,3Af', and IA", respectively (see Figure 3). All calculations were carried out using the GAUSSIAN-86package of codes.36 The 6-31+G** basis set3' was chosen as it includes both polarization functions on all atoms and a diffuse sp set on carbon, particularly useful in treating anions. Geometry optimizations were performed at the U H F level using the gradient routines in the program, followed by UMP3 computation of the energy. Vibrational frequencies, and zero-point energies, were obtained at the U H F level, assuming a fully harmonic force field. Spin eigenvalues of all U H F wave functions correspond to the state listed with higher spin contamination of less than for the triplets and 0.2 for the two excited singlets. The optimized geometries of the ground state, as well as the various excited states, are listed in Table I. As mentioned above, the first excited triplet state is nonlinear, with both cis and trans geometries representing local minima. Comparison of the energies of the first pair of excited states, cis 3B2and trans 3Bu,shows that the former is the more stable of the two. This difference in energy is about 8 kcal/mol at either the U H F or UMP3 level, as displayed in Table 11. The r(CC) distance undergoes a lengthening of 0.13 A upon excitation. Indeed, a similar lengthening is apparent in all the excited states, not surprising in view of the occupation of ( 3 6 ) Frisch, M. J.; Binkley, J. S.;Schlegel, H. B.; Raghavachari, K.; Melius, C. F.; Martin, J. L.; Stewart, J. J. P.; Bobrowicz, F. W.; Rohlfing, C. M.; Kahn, L. R.; DeFrees, D. J.; Seeger, R.; Whiteside, R. A,; Fox, D. J.; Fleuder, E. M.; Pople, J. A. GAUSSIAN-86 Carnegie-Mellon Quantum Chemistry Publishing Unit: Pittsburgh, PA, 1984. (37) Hariharan, P. C.; Pople, J . A. Theor. Chim. Arm 1973, 28, 213. Chandrasekhar, J.; Andrade, J. G.; Schleyer, P. v. R. J . Am. Chem. Soc. 1981, 103, 5609.
1.060 1.101 1.117 1.120
TABLE 11: Relative Energies (kcal/mol) of Ground and Excited States of Acetylene and H C C geometry state pt gp EUHF EUUP3 ZPVE" expt
HCCH linear cis
'&+
Dmh
'B2 'B,
C2,
trans linear cis trans
'xu+ D,h C,
linear
'Au
cis trans linear linear bent linear bent
linear bent linear
C2,
3,411
3Ajf
C, D,h C2,
'A2
'A,
c2h
D,h
0.0 61.84 69.22 115.39 85.30 80.06 134.41 96.84 88.26 141.66
0.0
83.77 91.93 140.32 111.71 101.30 159.17 117.58 108.21 166.03
18.37 16.30 16.51
0 60-10830 12226-28.30
15.28 15.41 1 3628 14.44 15.12
121''
HCC-
'E'
C ,, C, C,,
'A'
3x+
C,
'A" 'A 'A"
C ,,
C, C ,,
'E-
0.0 88.82 1 19.54b 76.77 97.32 131 .59b 82.76 103.85 140.15b 0.0 69.62
9.86 7.79 7.26 7.10
Zero-point vibrational energy computed at harmonic UHF level. Computed by PEPCI procedure at ground-state geometry.
-
TABLE 111: Deprotonation Energies (kcal/mol) of Acetylene
HCCH geometry state
---
linear
ICg+
cis trans
'B2 3A''
trans
'A,
HCCUMP3 + geometry state UHF UMP3 AZPVE linear
bent bent bent
IC' 385.8 'A' 393.6 3A'f 382.5 'A" 380.3
387.4 392.4 383.4 383.0
378.9 383.9 315.3 375.0
a C=C antibonding MO. A smaller stretch is also observed in the C-H bond lengths. The next highest level might be expected to be cis 3A2and/or trans 3Au. However, upon optimization of these symmetrical states, a single imaginary frequency was obtained for each. After the symmetry conditions were removed, each species relaxed to a C, structure of lower energy. This relaxation lowered the energy of the trans structure by 0.2 kcal/mol and the cis structure by 1.8. As shown in Table I, the two CH bond lengths in the cis geometry differ by 0.013 A. The O(CCH) bond angles are also unequal, differing by 24'. Similar inequalities, albeit not as severe, are observed in the trans structure. Table I1 shows that the trans 3Aff state is lower in energy than its cis analogue by 5-10 kcal/mol. About 7-8 kcal/mol higher in energy lies the next pair of states, the lowest excited singlet. Again, the trans arrangement ('A,) is favored over the cis (IA2), this time by about 9 kcal/mol. Some earlier theoretical studies are germane. D e m o ~ l i nused ~~ a frozen core restricted CI approximation incorporating single excitations from the IT,, M O to construct determinants. Geometries were chosen rather than optimized for the most part. A [7s3p/4s] basis set was supplemented by diffuse double-< functions
Excited States of Acetylene TABLE IV: Mulliken Atomic Charges in HCCH and HCCHCCH geometry state H c C linear ‘Eg+ 0.254 -0.254 -0.254 cis ’B, 0.149 -0.149 -0.149 trans ’B“ 0.161 -0.161 -0.161 cis 3Alf 0.151 0.033 -0.297 trans 0.156 -0.104 -0.186 0.158 -0.158 -0.158 cis “42 trans ‘A” 0.156 -0.156 -0.156
The Journal of Physical Chemistry, Vol. 95, No. 25, 1991 10283
HCC-
H
geometry
0.254 0.149 0.161 0.1 12 0.134 0.158
linear bent bent
0.156
bent
H
C
C
’A’
0.131 0.039
-0.179 -0.200
-0.953 -0.839
3Aff
0.007
-0.309
-0.698
A!f
0.016
-0.331
-0.685
state
‘E+
I
of the following types: s, p, and a partial set of d functions. Schaefer and c o - w ~ r k e r semployed ~ ~ , ~ ~ large-scale CI including all single and double excitations in conjunction with a DZP basis set. Geometry optimizations were stepwise rather than via gradient procedures. The excitation energies in Table I1 fall nicely in between the earlier computed results. For example, Wetmore and S ~ h a e f e r ~ ~ obtained an excitation energy of 80.5 kcal/mol for the 3B2C2, cis state as compared to Demoulin’s 8 1 .9.32 Our own value lies at 83.8 kcal/mol. In contrast, the R Y F excitation energies computed by Martin et al. with a small ir.flcxible 3-21G basis set35 or with the larger 6 - 3 1 C ~ * *are ~ ~smaller t l i x those obtained with the correlated methods above. These R H F values are, however, higher than the U H F results listed in Table 11, computed with our substantially more flexible basis. Another point of agreement is the energy differences computed between the cis and trans geometries of each state. Wetmore and Schaefer found that the cis 3B2geometry is more stable than the trans 3B, by 8.1 k ~ a l / m o l in ; ~comparison, ~ the energy difference obtained here is 8.2. A similar comparison of the cis-trans energy difference of the 3A states yields 8.8 kcal/mol by Wetmore and Schaefer and 10.4 here. The singlet state separations are 1l.134 and 9.4 kcal/mol, respectively. A vertical excitation from the ground state of HCCH, would involve a transition to the same linear geometry. The energies of these linear excited states are reported along with those of the optimized geometries in Table I1 and are considerably higher than those of the corresponding geometry-relaxed bent conformations. These relaxation energies appear to be as large as 60 kcal/mol. The U H F vertical excitation energies are in surprisingly good agreement with the experimental values listed in the last column of Table 11, although the UMP3 energies are considerably higher. We now turn our attention to the HCC- anion. Analogous to the HCCH molecule from which it is derived, the ground-state geometry is linear but electronic excitation produces a bent structure belonging to the C, point group. These excitations are again of R R* type where the latter antibonding M O has been substantially stabilized by the bending. As indicated in Table I, the CEC bond undergoes a stretch upon excitation of the anion similar to that observed in the neutral HCCH. The C-H bond elongates by a lesser amount but this stretch in the anion is considerably larger than that in the neutral. It is notable that the B(CCH) bond angle in the first excited state of HCC- is 127.3’, nearly identical to the value of 128.9’ in the corresponding first excited (3B2) state of HCCH. The angle in the next state of HCC- is 112.8’, smaller than either the 128.0’ or 116.8O values in the 3A” trans geometry of HCCH, likewise for the singlets where B(CCH) is 10’ smaller in the anion than in the neutral trans ’Au. It may be seen in Table I1 that the lowest triplet state is nearly 90 kcal/mol higher in energy than the ground state of HCC-, 70 at the U H F level. The next triplet is about 8 kcal/mol higher in energy, followed by the singlet which is higher by another 6 kcal/mol or so. It should be underscored that these excitation energies are not much different from those for neutral HCCH. Another important observation concerns the influence of correlation. The UMP3 excitation energies in Table I1 are greater than
the U H F values but nearly uniformly so. That is, the differences between the UMP3 and UHF excitation energies are all 19 f 3 kcal/mol. The penultimate column of Table I1 contains the total zero-point vibrational energy of each species. Consistent with the excitation to a R* MO and the attendant weakened CEC bonding, the vibrational energies are all smaller for the excited states. Indeed, the zero-point energies appear to become progressively smaller as the excitation energy increases for either HCCH or its anion. Table 111 lists the energies required to remove a proton from HCCH in each of the various states. As indicated in the first row of the table, removal of the proton from the ground state of HCCH, yielding the ground state of HCC-, requires 385.8 kcal/mol at the U H F level; the UMP3 result is only 1.6 kcal/mol higher. After correcting for zero-point vibrations, the UMP3 deprotonation energy is 378.9 kcal/mol. By incorporation of ApV and a translational correction, one obtains a AH’(298 K) of 380.4 kcal/mol, in reasonable agreement with the experimental measurement of 375.4 kcal/mol.’ The l % overestimate is not surprising in view of the better ability of the basis set to treat the neutral molecule as compared to the anion where the concentrated charge generally requires a more extended set of functions. A similar transition between the lowest energy triplet states of the two moieties requires 5 kcal/mol more energy than does ground state deprotonation whereas the the ground state 3A”(HCCH) 3A”(HCC-) deprotonation energy is smaller in magnitude by about 4 kcal/mol. Removal of a proton from the singlet state requires 375 kcal/mol, nearly identical to the latter triplet transition. In all the preceding deprotonation triplet reactions, the more stable of the cis vs trans conformations of H C C H has been chosen. Deprotonation of the higher energy conformer would require correspondingly less energy. In summary, it appears that the electronic excitation of the HCCH molecule has little influence upon its deprotonation energy, provided the final state of the HCC- corresponds to the initial state. This behavior stands in interesting contrast to the protonation energy of HCCH (and several of its substituted derivatives), which increases by up to 70 kcal/mol upon going from the ground to an electronic excited ~ t a t e . ~ ~ ” ~ As prior work with proton-transfer reactions has proven the value of analysis of charge distributions of the H-bond partners, it was deemed worthwhile to consider the atomic charges in the various states of both HCCH and HCC-. The Mulliken charges, computed a t the U H F level, are compiled in Table IV for each of the states considered here. Considering first the neutral HCCH in its ground state, about of a unit charge is assigned to each atom, H positive and C negative. Electronic excitation has the effect of lowering the positive charge on the hydrogen atom and to similarly reduce the negative charge on carbon. This transfer of electron density from C to H is sensible in light of the character of the R* orbital into which the electron is being excited. As a result of the bending, the hydrogen orbitals make a sizable contribution to this MO. For example, the s and p orbital coefficients in the bz(R*) M O for the hydrogen are equal to 0.13 and 0.61, respectively, when the B(CCH) angle is 140’. In contrast, the R M O acquires very little hydrogen population as the H coefficients are less than 0.05. This sort of C to H transfer was also
(38) Martin, P. S.; Yates, K.; Csizmadia, 1. G. Tbeor. Chim.Acta 1983, 64, 1 1 7. Yates, K.; Martin, P. S.;Csizmadia, 1. G.Pure Appl. Cbem. 1988, 60, 205.
2178.
-
--
-
(39) Martin, P. S.; Yates, K.; Csizmadia, 1. G. Can. J . Chem. 1989, 67,
J. Phys. Chem. 1991, 95, 10284-10294
10284
D.34
of the first triplet state is only some 5 kcal/mol (1%) higher than that of the ground state. The next triplet and first singlet require 3-4 kcal less energy to deprotonate than the ground state. These excitations cause a qualitative change in molecular shape as the excited states of both HCCH and HCC- are bent rather than linear. The energy differences between the cis and trans configurations of HCCH are of the order of 8-10 kcal/mol. Relaxation energies that take the excited state from the ground-state linear geometry to the structure optimized for the state itself are perhaps 60 kcal/mol. The molecular bending that accompanies each electronic excitation is responsible for a sizable redistribution of electron density. Occupation of the “a-type” x* virtual MO draws density from carbon toward the hydrogen atoms. The less positively charged hydrogen leads one to expect the electronically excited (bent) HCCH molecule to be a poorer proton donor than the linear ground state in any H bond. This notion conflicts with the purely energetic finding of a nearly identical deprotonation energy in the excited state. Similarly, the less negative C atom may act as a poorer proton acceptor. The situation is probably further complicated by other aspects of the nonlinear distortions. For example, another consideration is the dipole moment of the cis geometry. This moment, absent in the linear ground state, can profoundly influence the energetics of interaction with another molecule, especially if the latter is electrically charged.’6-20 The H-bond energy and energetics of subsequent proton transfer are thus subject to strong electrostatic influence and its associated angular dependence. Future work treating molecular complexes involving these species will examine if these presumptions are correct and the implications for proton transfer.
Conclusions The calculations reported herein indicate that the deprotonation energy of HCCH is not very much changed when electronically excited to its first few valence states. The deprotonation energy
Acknowledgment. We are grateful to the N I H for financial support of this research (Grant GM29391) and to Prof. Michael Kasha for support and helpful discussions. Registry No. Acetylene, 74-86-2; acetylide, 29075-95-4.
evident in the calculations of Wetmore and Schaefer for the lower lying triplets studied there.33 It is interesting to see how the unsymmetrical distortion of the second triplet state affects its charge distribution. Apparently the very small O(CCH) bond angles of the right side of the HCCH molecule lead to an even smaller positive charge on the hydrogen. The adjacent C atom has a surprisingly larger negative charge, with the left carbon losing a good deal of density. Further evidence that the charge shift is a direct result of the bending, as opposed to the electronic excitation itself, comes from computation of the charge distributions in the excited states, but maintaining the original HCCH ground-state geometry. Such an excitation produces a change of less than 0.03 in the atomic charges as compared to the ground state. In the ground state of HCC-, the bulk of the negative charge resides on the right C atom (the one not bonded to the hydrogen). The other carbon is slightly negative, and the hydrogen has a small positive charge associated with it. Excitation pulls density out of the right carbon. Some of this density is transferred to the other C and some to the hydrogen. Again a relationship is noted between this charge transfer and the O(CCH) bond angle in that the shift is magnified by the smaller angles in the two higher energy states. Another measure of the charge distribution is the total dipole moment of those geometries for which the moment is not forced to be zero by symmetry constraints. The dipole calculated at the U H F level for the lower energy cis 3B2state is 1.29 D, quite close to the value of 1.32 D calculated by Wetmore and Schaefer using a CI procedure.33 The moments of the higher energy states are considerably higher: 2.88 D for the cis 3A” state and 2.58 D for ‘A2. The latter values also agrees well with the CI result of 2.52
MO Study of Singlets, Triplets, and Tunneling in Tropolone. 1. Geometries, Tunneling, and Vibrations in the Ground Electronic State Richard L. Redington* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409
and Charles W. Bock Department of Chemistry and Natural Science, Philadelphia College of Textiles and Science, Philadelphia, Pennsylvania 191 44, The American Research Institute, Molecular Sciences Division, Upper Darby, Pennsylvania 19082, and Fox Chase Cancer Center, Philadelphia, Pennsylvania 191 44 (Received: March 12, 1991) The geometries of tropolone and the C, transition state separating its equivalent H-bond conformationshave k e n fully optimized using the 6-31G, 6-31G*, and 6-31G** basis sets. The minimum energy valley connectingthe equivalent H-bond conformations incorporates significant heavy atom displacements, and the barrier maximum computed using the 6-31G** basis set is 15.67 kcal/mol. The low and experimentally improbable value of 4.06 kcal/mol was obtained when the barrier height was computed at the MP2/6-31G*//RHF/6-31G1 level. The vibrational spectrum of tropolone in the So state was computed using the 6-31G basis set and a new vibrational assignment encompassing all 39 fundamental modes is presented on the basis of comparisons of the computed spectrum with the available spectroscopicexperimental data. Vibrational state-specifictunneling phenomena of tropolone are discussed in view of the new computations. The MO computations include investigation of the non-hydrogen-bonded trans internal rotation isomer of tropolone (6-31G, 6-31G*, and AMI) and tropolone monohydrate (AMI). AM 1 computations of tropone and 2-methoxytropone were also performed to investigate the stabilization of planarity in the seven-membered ring by the x electron system.
I. Introduction Tunneling transformations involving hydrogen bonds in complex molecules occur widely in chemical and biological systems, and their conceivable consequences are very far-reaching as can be 0022-3654/91/2095-l0284$02.50/0
seen from the discussion of D N A base pairing by L0wdin.l Recent investigations considering the characterization and mod(1) Lowdin, P.-0. Rev. Mod. Phys. 1963, 35, 724.
0 1991 American Chemical Society
