Electronic structure of hydroxyl-vinoxy radical HOCHCHO

Volume 165, number 1

CHEMICAL PHYSICS LETTERS

ELECTRONIC STRUCTURE OF HYDROXYGVINOXY

5 January 1990

RADICAL HOCHCHO

P. MOUGENOT and M. DUPUIS IBMCorporation, Data System Division, Department 48B/MS 428, Neighborhood Road, Kingston, NY 12401. USA Rec&ed 23 February 1989; in final form 6 November 1989

We report on the electronic structure of the hydroxyl-vinoxy HOCHCHO radical. Its ground electronic structure is shown to result from the interaction of two main resonating valence bond configurations. Computationally we used multiconfiguration SCF wavefuncrions to take into account the resonance interaction. We predict the existence of a near-infared electronic excited state (‘A’) about 1.0 eV above the ground state ‘A”, and of an excited state (‘A” ) in the LV regionat about 3.5 eV above the ground state.

1. Introduction The possibility

of using hydrogen-bonded

systems

by Aviram, Ratner and Seiden ( ARS) several years ago [ 11. ARS selected a hemiquinone as a model system for the purpose of studying the electronic structure characteristics for such a process. For completeness the molecular species involved are shown in fig. 1. ARS carried out ab initio calculations which overall showed unsatisfactory agreement with known experimental information. The recent report by Aviram et al. [ 21 of the possible rectifying behavior of an asymmetric double hydrogen-bonded system prompted us into reinvestigation the electronic for “dynamic

information

storage” was proposed

structure of this prototype hemiquinone system. In this paper we report on the electronic structure of the radical species involved in the single proton transfer. This is the hydroxyl-vinoxy radical HOCHCHO. Starting from an analysis of the valence bond structures of the radical, we designed multiconfiguration self-consistent-field (MCSCF) wavefunctions which yielded the structure of the ground state with ‘A” symmetry, of an excited state with 2A’ symmetry and transition energy near the infrared at about 1.OeV, and of a second excited state with ‘A” symmetry in the UV region at about 3.5 eV. The paper is organized as follows: section 2 contains a qualitative description ofthe electronic structure of HOCHCHO, including the definition of resonating valence bond structures which make a significant contribution to the radical structure. The results of the computer characterization of these valence bond structures by means of open-shell Hartree-Fock (ROHF-SCF) and generalized valence bond (GVB) calculations are presented in section 3. The resonance mixing of the valence structures by means of MCSCF calculations is reported in section 4.

2. Electronic structure of HOCHCHO ( 1.2 1

( Ia ,

Fig. 1,Theoretical model for the hemiquinone study.

The ground state and the first excited state with ‘A” symmetry of HOCHCHO result from the reso-

0009-2614/90/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland )

87

Volume 165, number 1

Fig. 2.

Valence

bond structures ofOH-CHCHO.

nance interaction of the hydroxyl-ethenyloxy structure HO-CH=CH-0’ and the hydroxyl-methyl-formy1 structure HO-C’H-CH=O. Valence bond pictures of these two resonance structures are shown in fig. 2. We consider first the hydroxyl-ethenyloxy structure (Za). It has a C=C double bond, an “out-ofplane” x unpaired electron located on the oxygen atom, an “out-of-plane” lone pair of electrons on the oxygen atom of the hydroxyl group, and an “inplane” o lone pair of electrons on the aldehyde oxygen and therefore conforms to ‘A” symmetry. This structure can be characterized by means of calculations using ROHF-SCF and GVB wavefunctions. We consider now the hydroxyl-methyl-formyl structure (2b). It has an “out-of-plane” tt unpaired electron located on the carbon atom to which the hydroxyl group is attached, a C=O double bond, an “out-of-plane” lone pair of electrons on the oxygen atom of the hydroxyl group, a o lone pair of electrons on the aldehyde oxygen atom and therefore also has ‘A” symmetry. This structure too can be characterized with SCF and GVB calculations. Although the two resonance structures just mentioned have quite distinct bonding characteristics, their geometries arc not so extremely diffcrcnt, and thus their wavefunctions have nonzero overlap. It follows that if we consider a wavefunction which has the form of a linear combination of these two resonating wavefunctions, to which we apply the vari88

5 January 1990

CHEMICAL PHYSICS LETTERS

ational principle, we can find a set of coefficients corresponding to a wavefunction with an energy lower than either of the energies of the valence structures. The stabilization or resonance energy increases with the overlap of the two resonating structures. The destabilization of the orthogonal excited state increases even faster with the overlap of the two resonance structures. The correlation diagram of the resonance interaction is depicted in fig. 3. The MCSCF formalism, as its name indicates, provides a convenient means to calculate the resonance mixing of these two structures as outlined in section 4. In addition to the ‘A” structure described above, the radical HOCHCHO has an 2A’ structure shown in fig. 2d. It differs from the ‘A” structure (2a) in the occupation of the o and n orbitals located on the oxygen atom of the oxy group. In the ‘A’ structure, the oxygen lone pair occupies a E orbital, while the unpaired electron occupies a o in-plane orbital There exists no other low-lying 2A’ structure analogous to (2b), with which this ‘A’ configuration can strongly interact. The valence structure (2d) can be adequately characterized with SCF and GVB calculations.

kcol/mol

f

\ \

‘*,I

, \

RHF

'\

, , '

I ,

GVB -40

I' RHF ,'

;A' '-26.0

-24.3", J

/-x.3

.\,

8'

,,'

GV8

I&Y CAS

Fig. 3. Correlation diagram for the ‘A” and ‘A’ states of the HOCHCHO radical.

Volume 165,number 1

CHEMICALPHYSICSLETTERS

3. The HF-SCF resonance structures The HF model yields two local minima on the *A” potential energy surface corresponding to the two valence structures described earlier. Only one minimum is found on the ‘A’ hypersurface. The most important geometrical parameters of these structures are given in table 1 while the total and relative energies are given in table 2 for the 3-21G basis set [3,4] #I. For comparison with the results given in table 1, we note that the 3-2 1G basis set give d( C=C) = 1.3 15 A for ethylene, d(C-C) = 1.542 A for ethane, d(C=0)=1.209 A and d(C-C)=1.507 A for acetaldehyde, and d(C-0) = 1.440 A and d(O-H) =0.966 A for methanol. On the basis of these results, we are justified in assigning the local minimum energy structures to the hydroxyl-ethenyloxy and hydroxyl-methyl-formyl radical structures respectively. In the ‘A” hydroxyl-ethenyloxy structure (2a), the C=C bond length ( 1.329 A) corresponds to an almost pure double bond as in ethylene, while the oxy C-O bond length (1.364 A) is much longer than the C=O bond of acetaldehyde, and somewhat shorter i(’All calculationswereperformedon an IBM 3090computer withthe HONDOprogram.For HONDO,seeref. [ 41.

5 January 1990

than the C-O single bond of methanol. These features can be attributed to the interaction of the unpaired electron on the 0 atom with the x bond. In the other resonance structure of ‘A” symmetry, the hydroxyl-methyl-formyl structure (2b), the oxy C=O bond length ( 1.232 A) corresponds to an almost pure double bond as in acetaldehyde, while the C-C bond length is clearly longer than a normal C=C double bond, but shorter than the C-C bond in ethane. Here also the unpaired electron on the carbon atom is responsible for the slight relaxation of the double bond in a p position to the radical center. We note also that the C-OH bond (1.352 A both) and the O-H bond are nearly the same in both structures. The 2A’structure (2d) ofthe hydroxyl-ethenyloxy radical is similar to the *A” structure. The out-ofplane orientation of the lone pair on the oxygen of the aldehyde group results in a greater electron repulsion with the C=C bond and is responsible for a shorter C=C bond and a longer CO bond. The hydroxyl group in the ‘A’ structure has parameters similar to those of the same functional group in the 2A” structures. In order to separate electron correlation effects in the x bonds and the resonance mixing effects, all of which are included in the MCSCF calculations described in section 4, we decided to carry out GVB

Table I HF and GVB-PP optimized resonance structures ‘) of HOCHCHO Resonance structure, states

Hydroxyl-ethenyloxyradical

Hydroxyl-methyl-formyl radical, ‘A” 2A”

*A’

RHF

GVB

RHF

GVB

RHF

GVB

bond length G-0, G-H, G-C, Cw-f-L G-0, 0,-H,

1.406 1.066 1.310 1.069 1.381 0.966

1.407 1.066 1.329 1.069 1.386 0.966

1.365 1.067 1.329 1.068 1.352 0.971

1.403 1.066 1.329 I.068 1.364 0.970

1.232 1.079 1.419 1.065 1.352 0.973

1.229 1.078 1.464 1.063 1.366 0.971

angle 0,-C& C&-O, &-0,-H,

124.5 127.2 113.1

124.0 126.7 113.2

111.7 123.6 110.4

117.7 122.8 110.4

120.5 118.1 109.6

120.6 117.3

110.0

‘) Bond lengths are in A, angles in deg. The labels are in accord with the formula H&O,-C,H&H,O,.

89

Volume 165,number 1

5 January 1990

CHEMICALPHYSICSLElTERS

Table 2 Total a) and relative b, energies of the SCF, GVB-PP optimized resonance structures, and of the MCSCF ground and excited states of HOCHCHO Methyl-formyl structure, ‘A”

Ethenyloxystructure 2* I RHF GVB

0.877306 (- 13.4) 0.894748 (-24.3)

0.855961 (0.0) 0.884366 (-17.8)

Excited state, ‘A”

Ground state

MCSCF

0.875539 (- 12.3) 0.897317(-26.0)

**II

2A’

0.918518 (-39.3)

0.885156 (- 18.3)

0.793350 (+39.7)

a) Total energies in au shifted by + 225. b, Relative energies in kcal/mol are given in parentheses.

calculations of the three structures in the perfect pairing (PP) approximation. In these calculations we include a pair excited configuration for each bond which is formally a double bond. These calculations thus take into account the n’-n* correlation effects which of course are not included in the SCF calculations. Just as in the SCF calculations, three local minima were found with GVB-PP on the potential surface. They resemble the SCF resonance structures and are listed in table 1 also. The GVB-PP wavefunction reinforces the single and double bond character of the various bonds. From these calculations we obtain an estimate of the X-K* correlation energy as the difference between the SCF energy and the GVB energy for each structure: 17.8 and 10.9 kcal/ mol for the 2A’ and *A” hydroxyl-ethenyloxy structures respectively, and 13.7 kcal/mol for the ‘A” hydroxyl-methyl-formyl structure.

4. Ground and excited state of HOCHCHO From the above qualitative description of the valence states of the hydroxyl-vinoxy radical, it is easy to construct compact wavefunctions which can describe the resonance interaction of the valence structures of HOCHCHO. These wavefunctions are of the complete active space (CAS) MCSCF type [ 51, For the 2Aa states we consider the four n orbitals built from the four atomic PITout-of-plane functions. Twenty configurations are generated by distributing the live n electrons among the four R orbitals in all 90

possible ways consistent with spin and spatial symmetry. Among them one finds the configurations which make up the SCF and GVB-PP wavefunctions of the valence structures. In addition one finds the spin recoupling configurations which are the essence of the resonance mixing, as discussed later. For the ‘A’ sixteen configurations are generated by distributing six electrons among the four n orbitals and keeping an unpaired electron in the oxygen o orbital. Among these one finds the SCF and GVB-PP configurations, which happen to be the most important ones for this state. The MCSCF calculations go beyond the GVB calculations in the sense that in addition to the ~~--n*~ correlation effects, the MCSCF wavefunction allows for mixing of the resonance configurations. The configurations in which the unpaired electron recouples its spin with the spins of the x electrons, are the key ingredient which yields resonance stabilization. More explicitly, in the GVB wavefunction the two 7[bond electrons are coupled in a singlet spin, and are further coupled with the unpaired electron into a doublet state. In the MCSCF wavefunction the resonance effects are described by means of the configuration in which the two x bond electrons are coupled in a triplet spin, and then are further coupled with the unpaired electron into a doublet state. This is the spin recoupling process. The most important geometrical parameters of the MCSCF optimized structures are given in table 2. Total and relative energies are presented in table 3. In table 4 we give a chemical description of the nat-

Volume 165, number 1

CHEMICAL PHYSICS LETTERS

Table 3 MCSCF optimized structures ‘) of the ground and excited states of HOCHCHO X2.4”

a’A’

I ‘A”

G-Hb G-0, 0,-H,

1.278 1.074 1.394 1.066 1.362 0.970

1.406 1.066 1.329 1.069 1.385 0.966

1.397 1.066 1.462 1.063 1.375 0.969

angle o,-c,-c, c,-c,-0, &-O.-H.

120.5 120.7 110.4

123.8 126.5 113.1

117.6 118.2 110.5

State bond length GO, G-H, c,-c,

a) Bond lengths are in A, angles in deg. The labels are in accord with the formula H,O,-CbHb-C,H,O,. Table 4 Occupation numbers ofthe fractionally occupied natural orbltals of the MCSCF wavefunctions of HOCHCHO State *A’state

‘A” states

Orbital

Character

Occupation

I a”

0 (lone pair)

2a” 3a” 4a” a’

0 (lone pair) A c-c K’c-c unpaired e

1.999 1.999 1.915 0.086 1.000

1a” 2a” 3a” 4a” a’

c2tc,to, 0 (lone pair) q-0, c&+0, 0 (lone pair)

ground 1.318 1.998 1.128 0.565 2.000

excited 1.904 1.998 1.000 0.094 2.000

ural orbitals of the MCSCF wavefunctions along with their occupation numbers. The structure of the ground state 2A” is nearly half-way between the two interacting resonance structures with a slight predominance of the hydroxyl-methyl-formyl mesomer. The first excited state has a significantly different structure compared to the ground state. The C-C bond and the oxy C-O bond are much longer ( 1.462 and 1.397 .A) than in the ground state ( 1.394 and 1.278 .A). These differences can be explained by considering the character and occupation of the natural orbitals as reported in table 4. The orbitals with one and two nodal surfaces have higher occupation number in the excited state than in the ground state resulting in longer C-C and C-O bonds.

5 January 1990

A comparison of tables 1 and 3 shows that the 2A’ of HOCHCHO has nearly identical GVB and MCSCF structures. For this state the role of the configurations included in the MCSCF wavefunction is merely to describe electron correlation since no resonating structure exists. The most important configurations are the same ones found in the GVB-PP wavefunction, and therefore the two optimized structures are very much alike. Comparison of the GVB-PP and MCSCF energies yields the resonance stabilization energy for the X ‘A” state. It amounts to about 13.0 kcal/mol. We note that the calculated resonance stabilization energy reported here is of semi-quantitative accuracy only since as reported by Baird et al. [6,7] the 3-21G basis set does not describe very well the relative strength of the K components of C=C and C=O double bonds, and a double-zeta plus polarization basis set would be necessary to stabilize the oxygen lone pairs by bonding participation through the hydrogen atoms. However, on the basis of calculations reported previously for the vinoxy radical [ 8 ] CH,CHO, we believe that the present work takes into account all the major effects which enter into the electronic structure of this system. We expect the term energies for the *A’and 2A” states to be around 1.0 and 3.5 eV. state

5. Conclusions We have presented a qualitative description of the most important valence bond effects which play a key role in defining the electronic structure ofthe hydroxyl-vinoxy radical. These features were corroborated by SCF, GVB-PP and MCSCF calculations. From this study we predict the existence of a nearinfrared electronically excited state, and of an UV excited state for the HOCHCHO radical. Further characterization of the potential surface for the dimer radical is in progress and will be reported soon. This further work relies on the findings reported here. Finally we note that the recent experimental work of Aviram et al. [2] dealt with .a system in which the hydroxyl and oxy groups are attached to a six-membered ring. We believe that the findings presented in this paper will apply to these systems as well, although the additional z bonds are 91

Volume 165, number

I

CHEMICAL PHYSICS LETTERS

likely to change somewhat the term energies of the excited states. References ] A. Aviram and P.E. Seiden, US Patent No. 3,833,894 ( 1974); A. Aviram, P.E. Seiden and M.A. Ratner, 25th IUPAC Congress, Abstact (1975) p. 195; IBM internal publication RCNo. 5919 (1976). ] A. Aviram, C. Joachim and M. Pomerantz, Chem. Phys. Letters 146 ( 1988) 490. ] J.S. Binldey, J.A. Pople and W.J. Hehre, J. Am. Chem. Sot. 102 (1980) 939.

92

5 January 1990

[ 41 M. Dupuis, J. Rys and H.F. King, J. Chem. Phys. 65 ( 1976) 111; M. Dupuis. J.D. Watts, H.O. Villar and G.J.B. Hurst, Computer Phys. Commun. 52 ( 1989) 4 15; M. Dupuis, P. Mougenot, J.D. Watts, H.O. Villar and G.J.B. Hurst, in: Modem techniques in computational chemistry, ed. E. Clementi (Escom, Leiden, 1989). [ 51B.O. Roos, P.R. Taylor and P.E.M. Siegbahn, Chem. Phys. 48 (1980) 15?. [ 61 N.C. Baird, R.R. Gupta and K.F. Taylor, J. Am. Chem. Sot. 101 (1979) 4531. [7] N.C. Baird and K.F. Taylor, Can. J. Chem. 58 ( 1980) 733, [8] M. Dupuis, J.J. Wendoloski and W.A. Lester Jr., J. Chem. Phys. 76 (1982) 488.