6-31G* structural study of tropone and tropolone molecules

Journal of Molecular Structure, 318 (1994) 217-235 0022-2860/94/$07.00 0 1994 - ElsevierScienceB.V. All rights reserved

211

MP2/6-3 lG* structural study of tropone and tropolone molecules N. Sannaa, F. Ramondob, L. Bencivennibs* ‘~onsorzio App~icazioni S~percalcoio, university di Roma ~~ASPUR~, c/o Centro Interd~artimenta~e Calcolo Scientifico, Universith degli Studi di Roma, PSe A. More 5,00185, Rome, Italy bDipartimento di Chimica, Universitci degli Studi di Roma, P.le A. More 5, 00185 Rome, Italy

di

(Received 6 August 1993) Abstract The effects of electron correlation on the molecuIar structure and stability of tropone and tropolone molecules have been studied by MP2/6-31G* calculations. Geometry optimizations have been carried out for tropone, ctitropolone and the non-hydrogen bonded orthogonal and trans rotamers of tropolone. The tropone sevenmembered ring exhibits an appreciable triene-like character which is slightly smoothed upon OH substitution. The asymmetry of the hydrogen bonding of tropolone, emerging from previous HF/6-3lG* studies, is reduced by inclusion of etectron correlation, and the proton tunnelling barrier height drastically lowers from 70 kJ mol-’ (HF/ 6-31G*) to 25 kJ mol-’ (MP2/6-31G”). The structural changes induced by OH torsion, supported by additional MP2/6-31G* geometry optimizations of the analogous systems 1,4-pentadiene-3-one and 2-hydroxy-l,cpentadiene-3-one indicate that intramolecular C=O . . HO hydrogen bonding is favoured by n-delocalization through the cyclic carbon skeleton. SCF calculations on the centrosymmetrical tropotone dimer indicate that self-association enhances ~-del~alization in the troponoid ring and causes vibrational frequency shifts which satisfa~to~iy agree with the changes observed in the FT-IR matrix spectra of isolated tropolone molecules and of solid samples.

Introduction The geometry of molecules having non-benzenoid seven-membered rings have attracted noticeable interest in connection with their aromaticity. The alternation of the bond order of the ring carboncarbon bond distances of tropone (2,4,6cycloheptatriene-l-one) and tropolone (2-hydroxy-2,4,6-cycloheptatriene-l-one) molecules, providing a probe of 7r-delocalization, has been the subject of a wide number of experimental [I -1 and theoretical [8-101 studies. The tropone molecule (Fig. la), the prototype of troponoid systems, was investigated by X-ray studies of the crystalline solid [l], by gas-phase electron diffmction [2] and ~~owave [3] studies of the * Corresponding

author.

SSDI 0022-28~93)07904-B

free monomer, by proton magnetic resonance measurements of the monomer dissolved in a nematic solvent [4] and, more recently, by ab initio molecular orbital calculations [8]. From these studies it emerges that tropone is a planar molecule with alternating short and long CC bond distances. The tropolone molecule (Figs. Ib, lc and ld), where the H(C2) atom is replaced by an OH group, was the subject of several structural [5,6,9, lo] and spectroscopic [7, lo] investigations and particular emphasis was given to the intramolecular hydrogen bonding. The X-ray data analysis [6] and the recent ab initio study [lo] indicate that the crystal, as well as the free tropolone molecule, have a planar ring and the triene character appears to be less outstanding than that determined for the tropone molecule. Such considerations led

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et i&/J. Mol. Struct. 318 (1994) 217-235

a

b

d

Fig. 1. Numbering

of atoms in tropone (a) and in cis- (b), trans- (c) and C,, (d) symmetry structures of tropolone.

to the conclusion that tropolone is more stable than tropone and the extra stability was attributed to the increasing contribution of the dipolar canonical forms to the ground state of the molecule [6]. Notwithstanding the wide number of structural investigations, unambiguous conclusions relating to the aromaticity of the troponoid ring and the

effects of the introduction of the OH substituent on the ring stability have still not been entirely reached. The X-ray geometries of tropone [l] and tropolone [6] do not readily provide the intrinsic structural features of the troponoid system since they are related to the crystal molecule which may be appreciably deformed with respect to the

N. Sanna et al./J. Mol. Strut.

318 (1994) 217-235

219

a

e Fig. 2. Selected geometrical parameters (bond distances, A and bond angles, deg) of 1,4-pentadiene-3-one, (a) HF/6-31G* and (b) MP2/6-31G*, of cis-2-hydroxy-1,4-pentadiene-3-one, (c) HF/6-31G* and (d) MP2/6-31G*, and of trans-2-hydroxy-l,4pentadiene-3-one, (e) HF/6-31G* and (f) MP2/6-31G*. molecule [ 1l- 141.This effect is expected to be particularly outstanding for crystal tropolone where the molecules are hydrogen bonded pairwise to form ring dimers through C=O.. . HO hydrogen bonds [6]. isolated

To obtain more accurate information, it would be necessary to dispose of experimental data of the free molecule. The gas-phase electron diffraction investigation of tropone [2] is at present the only one providing direct observations of the vapour-

220

phase molecule. This study unquestionably verifies the CC bond aIternation of the seven-mem~red ring, although the data analysis provides two sets of geometrical parameters fitting the observations equally well. However, no geometrical information of gas-phase tropolone has been reported, to our knowledge. An earlier electron diffraction study [S] suggests the planarity of the ring, however equal CC bond distances were assumed in the data analysis. Therefore, the more reliable source of information of the free molecule is found in the recent ab initio investigations on tropone [S] and tropolone [9, lo] molecules using the Hartree-Fock (HF) elf-consistent-meld (SCF) methods by employing polarized basis sets. However, more accurate molecular geometries are expected from ab initio methods including electron correlation corrections, in particular when molecules contain CC alternating single and double bonds [ 151or undergo intramolecular hydrogen bonding [ 161. For these reasons, the primary objective of the present work is to reinvestigate the structural features of the isolated troponoid system by geometry optimizations using the M0lIer-PIesset perturbation theory [ 171through the second order approximation (MP2) by employing the 6-31G* basis set [18]. Intramolecular hydrogen bonding and internal proton exchange of tropolone are discussed by MP2/6-3 1G* geometry optimizations of the internal rotation isomer (Fig. lc), obtained from the most stable conformer on a 180” turn of the C-O bond, and of the CZvsymmetry structure (Fig. Id), where the hydrogen bond is symmetric. The geometry and stability of the five-mem~red hydrogen bonding ring are discussed in relation with an analogous species, 2-hydroxy- 1,4-pentadiene-3one (Fig. 2) where the bond rr-system is extended onto a smaller and open carbon skeleton. In addition to the study of the isolated molecule, this paper deals wth the structural and vibrational perturbations due to intermolecular hydrogen bonding. Ab initio calculations assume an important role in the investigation of the crystal field effects. Reliable information about the structural effects due to intermolecular interactions have

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been recently provided from theoretical calculations for suitable models simulating the intermolecular interactions in the crystal phase [ 12- 141. This paper reports the result of a SCF study of the centrosymmetrical dimer of tropolone, thus simulating the dimer structure observed in the crystal [6]. In order to relate the frequency shifts to the structural modifications exerted by hydrogen bonding, the FT-IR spectra of the tropolone monomer, previously measured in a Ne matrix [7] and assigned on the basis of HF/6-31G normal coordinate analysis [lo], have been used as a reference to study the FT-IR spectrum of solid tropolone, where the molecules form ring dimers through C=O - . . HO hydrogen bonds.

Computational

details

The geometries of tropolone, tropone, 1,Cpentadiene-3-one and 2-hydroxy- 1,4-pentadiene-3-one molecufes were optimized at the second order of the Moller-Plesset theory with all orbitals active using the 6-31G* basis set by an analytical gradient, basic technique. The MP2 calculations were performed using fully direct methods [19]. Normal coordinate and IR band intensity studies were accomplished at the SCF level using the 4-31G* basis set for the tropone and tropolone molecules and the 6-3 1G basis set for the tropolone dimer. All computations were carried out using the.IBM-VM/ CMS version of the GAUSSIAN 90 package 1201running on an IBM 3090-6005 computer.

Experimental

The FT-IR matrix spectra were measured using a Bruker IFS-l 13V interferometer. Commercial samples ~Aldrich 98%) were vaporized at 280K from a suitable molecular source and were concentrapped, at different matrix/molecule trations, onto Ar and N2 matrices maintained at 12 K. Annealed matrix spectra were obtained by controlled wa~ng up to 35K. The solid film spectra were measured after condensation of the

N. Sanna et d/J.

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vaporized samples onto reflecting copper surface.

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318 (1994) 217-235

the low temperature,

Results and discussion Tropone molecule

As a preliminary study of the tropolone system, we have investigated the structural features of the non-substituted troponoid skeleton of the tropone molecule. The geometry of this species was determined at the SCF level in a recent ab initio study [8] using split-valence polarized basis sets. When compared with the experimental data [l-4], the ab initio geometries reproduce ~tisfacto~ly the structural features of tropone. In particular, the theoretical and experimental geometries (although they have

a different physical meaning and so can only be compared cautiously) show a pronounced sequence of carbon-carbon (CC) single and double bonds. The molecular structure of tropone was reinvestigated in this paper by MP2/6_31G* geometry optimizations carried out within the C,, symmetry constraint. The optimized geometry and the rotational constants computed at the MP2/6-31G* level are reported in Table 1 along with the previous SCF results [8] and the experimental data [l-3]. The salient feature of Table 1 is that electron correlation causes a conspicuous lengthening of the bond distances with prevalent double bond character and a simultaneous shortening of the CC single bonds with respect to the SCF values. As a consequence, the alternation between single and double CC bond distances is much less marked

Table 1 Geometrical parameter&’and rotational constants of the tropone molecule HF/3-21Gb Bond distance (2 j 1.224 Cl=0 1.467 ClC2 1.332 C2=C3 1.443 c3c4 1.334 c4=c5 1.073 C2H 1.075 C3H 1.073 C4H Bond angle (deg) 121.8 C7ClC2 131.0 ClC2C3 130.2 C2C3C4 128.0 c3c4c5 111.3 ClC2H 113.8 C4C3H 117.3 CSC4H Rotational constants (MHz)’ 3788.40 A 2051.81 B 1330.96 C

HF/6-31Gb

1.235 1.463 1.342 1.442 1.343 1.073 1.075 1.074 123.0 130.3 130.15 128.0 111.8 114.1 117.1 3762.96

2049.10 1326.67

HF/6-31G* b

1.204 1.475 1.336 1A49 1.337 1.075 1.077 1.076

Electron diffractiond

1.245 1.464 I.365 1.429 1.367 1.089 1.089 1.088

122.4 130.4 130.3 128.1 111.6 114.0 117.1

122.0 130.9 130.1 128.0 111.8 114.7 116.6

3746.86 2058.74 1328.69

3744.93 2011.19 1308.48

1.224(4) 1.475(2) 1.362(2) 1.429(2) 1.342(5) 1.096(S) 1.096(S) 1.096(5) 125 127.5 133 127

’ The least-square deviations are reported in parentheses, as units in the last digit. b Ref. 8. ’ This work. ’ Ref. 2. e Ref. 1, parameters are averaged values. f The experimental rotational constants are A = 3712.34, B = 2027.64 and C = 1313.14MHz, ref. 3.

1.225(4) 1.445(2) 1.364(2) 1.462(2) 1.342(6) 1.098(6) 1.098(6) 1.096(6) 122 133 126 129.5

X-raye

1.258(9) 1.448(9) 1.358(9) 1.42819) 1.337(9)

123 130.5 130.3 128

222

N. Sanna et al./J. Mol. Strut. 318 (1994) 217-235

Table 2 Average CC bond distances, (CC ) (A) of tropone and tropolone

HF/6-JIG* a (Cl -C2)b Al’ (C2=C3)d A2e (C3-C4)f A3g c4=c5 MP2/6-SIG* (Cl -C2)b Al’ (C2=C3jd AT (C3-C4)’ A3g c4=c5

molecules and bond distance differences, A (A)

Tropone

Orthogonal-tropolone

Truss-tropolone

Cis-tropolone

1.475 0.139 1.336 0.113 1.449 0.112 1.337

1.482 0.144 1.338 0.107 1.445 0.108 1.337

1.483 0.142 1.341 0.100 1.441 0.102 1.339

1.470 0.125 1.345 0.088 1.433 0.088 1.345

1.470

1.468 0.095 1.373 0.046 1.419 0.048 1.371

0.075 1.379 0.030 1.409 0.028 1.381

1.464 0.099 I .36.5 0.064 1.429 0.062

1.367

a From the optimized geometries b Average value between Cl -C2 ’ (Cl-C2) - (CZ=C3). d Average value between C2=C3 e (C3-C4) - (C2=C3). ‘Average value between C3-C4 s (C3-C4) - C4=C5.

0.101 1.369 0.055 1.424 0.055 1.369

1.454

of refs. 8 and 10. and Cl -C7 bond distances. and C6=C7 bond distances. and CS-C6 bond distances.

than that predicted at the SCF level, as clearly emerges from the values of the CC bond length differences summarized in Table 2. On the contrary, the bond angle values show no significant changes when electron correlation is taken into account. Since a complete and accurate experimental structural determination of the free molecule has not yet been obtained, the MP2 determination is, at present, the best available approximation of the equilibrium geometry of the tropone molecule. The quality of the MP2/6-3lG* structure is proved when the theoretical rotational constants are compared with the accurate experimental values obtained from microwave data [3]. From such a comparison it emerges that the MP2 optimized geometry provides rotational constants reproducing the experimental ones with an average deviation (0.6g%) smaller than that dete~ined at the SCF level using the 3-21G (1.53%), 6-31G (1.15%) and 6-31G* (1.22%) basis sets. This observation

suggests therefore that the MP2/6-31G* structure of tropone is more accurate than the SCF one. Inclusion of electron correlation causes structural changes implying that the r-electron cloud is more uniformly distributed onto the ring, but the MP2 as well as the SCF determinations clearly indicate the triene feature of the tropone ring. Tropolone molecule

The location of the proton between the oxygen atoms of the tropolone molecule has been a matter of theoretical [9,10] and spectroscopic [7] work. The most recent ab initio study reports the results of SCF geometry optimizations using the split valence 6-31G, 6-31G* and 6-31G** basis sets [IO] and indicates that the C, symmetry configuration (Fig. lb), where the proton is localized on a single oxygen atom, is energetically preferred to the symmetric C,, saddle-point structure (Fig. Id), where the proton is equidistant from each oxygen

a Ref. 10. b This work. ’ Ref. 6; the least-square

Bond angles (deg) C2Cl=O 118.7 ClC2-0 110.2 C20H 110.8 ClC2C3 128.7 C2C3C4 131.1 c3c4C5 129.4 C4CSC6 126.8 C5C6C7 130.2 C6C7Cl 131.8 C7ClC2 122.0 C4C3H 113.7 C5C4H 116.7 C6C5H 115.7 C5C6H 114.5 C6C7H 117.7 115.9 112.2 107.4 129.3 129.8 129.6 127.1 130.6 130.4 123.4 115.8 116.3 115.6 114.4 118.0

1.212 1.330 2.525 0.952 1.488 1.345 1.432 1.345 1.433 1.345 1.452 1.076 1.077 1.075 1.077 1.075 115.2 111.8 102.5 130.2 129.0 129.2 127.7 130.1 130.5 123.3 117.0 115.9 116.0 115.1 117.2

1.260 1.339 2.507 0.999 1.471 1.381 1.406 1.381 1.412 1.376 1.436 1.088 1.089 1.088 1.089 1.088 115.4(2) 114.7(2) 107(2) 128.8(3) 129.4(3) 129.9(3) 127.5(3) 129.4(3) 130.6(2) 124.2(2)

1.261(3) 1.333(3) 2.553 0.94(3) 1.454(4) 1.379(4) 1.393(4) 1.341(4) 1.410(4) 1.373(4) 1.410(3)

X-ray’

107.6 107.6 93.2 127.7 127.6 131.0 124.4 131.0 127.6 127.7 117.6 114.8 116.3 114.8 117.6

1.263 1.263 2.252 1.204 1.490 1.386 1.386 1.386 1.386 1.386 1.386 1.075 1.077 1.075 1.077 1.075

HF/6-31G*a

C,, transition

as units in the last digit.

MP2/6-31G*b

standard deviations are reported in parentheses

118.9 110.0 108.7 129.5 130.8 129.9 127.0 128.9 132.5 121.4 114.35 116.35 116.3 115.2 116.9

1.243 1.356 2.551 0.976 1.480 1.377 1.417 1.371 1.420 1.369 1.456 1.092 1.088 1.088 1.089 1.088

HF/6-31G*a

HF/6-3lG*’

MP2/6-31G*b

Cis isomer

of tropolone molecules

Trans isomer

Bond lengths (A) Cl=0 1.198 c2-0 1.339 o...o 2.541 OH 0.948 ClC2 1.497 C2=C3 1.343 c3c4 1.441 c4=c5 1.339 C5C6 1.441 C6=C7 1.339 C7Cl 1.469 C3H 1.079 C4H 1.076 C5H 1.075 C6H 1.077 C7H 1.075

Parameter

Table 3 Geometrical parameters

109.15 109.15 92.0 127.8 128.2 129.8 128.4 129.8 128.2 127.8 117.5 115.3 115.8 115.3 117.5

1.295 1.295 2.320 1.243 1.470 1.403 1.389 1.397 1.397 1.389 1.403 1.088 1.089 1.087 1.089 1.088

MP2/6-31G*b

state

119.0 113.0 111.0 129.0 131.1 128.8 127.3 130.1 131.5 122.0 114.8 117.0 115.4 114.4 117.7

1.076 1.076 1.076 1.077 1.075

1.473

1.200 1.361 2.613 0.949 1.490 1.338 1.445 1.337 1.445 1.337

HF/6-31G*b

Orthogonal

119.3 113.3 109.4 129.9 130.8 128.5 127.3 129.9 132.1 121.4 115.8 116.5 116.0 115.0 116.9

1.242 1.380 2.639 0.973 1.478 1.370 1.423 1.369 1.425 1.367 1.461 1.089 1.088 1.088 1.089 1.089

MP2/6-31G*b

isomer

224

atom. However, the barrier height relative to the internal proton exchange, the value of which is expected close to that of malonaldehyde (24-28 kJ mol-*) [21], is largely overestimated at the SCF level (70 kJ mol-’ from an HF/6-3 1G* evamation). In addition, it was noted that inclusion of electron correlation energy at the MP2 level, carried out at the HF/6-3 1G** optimized geometry, causes a large decrease in the barrier height which is 17.0 kJ mol-’ at the MP2/6-3 lG*//HF/6-3 1G* level [lo]. For the malonaldehyde molecule, Frish et al. [16] determined, from geometry optimizations including electron correlation, barrier heights of 15.1 kJmol_’ (MP2/6-31G**), 25.5 kJmol_’ (MP3/ ~3lG**//MP2/6-3lG**) and 18.0 kJmol-’ (MP4/ 6-3 1G**//MP2/6-3 1G**), reasonably close to the tunnelling height estimated from the observed tunnelling splitting (24-28 kJ mol-‘) [211. In addition, the theoretical research on malonaldehyde [16] demonstrated as the Hartree-Fock approximation is incapable of describing the equilib~~ geometry of the molecule accurately. Bearing in mind these results, it is worthwhile reconsidering the molecular structure of tropolone and the internal proton exchange by geometry optimi~tions including electron correlation corrections. The MP2/6-3lG* optimized geometries of the C, and C,, symmetry structures of tropolone obtained in this paper are compared with the HF/6-31G* results [lo] and the X-ray experimental parameters [6] (see Table 3). In agreement with the SCF calculations and with the X-ray data analysis [6], the present MP2/6-31 G* computations confirm that the ground state of the tropolone molecule has an asymmetric hydrogen bond. By analysing the relative stability between the C, and C,, symmetry structures, it emerges that the second-order perturbation theory drastically reduces the barrier height of internal proton exchange from 70.1 kJmol_’ (HF/6-31G*) [lo] to 25.8 kJmol-’ (MP2/6-31G*). Such a result is consistent with the improvement of the size of the barrier of malonaldehyde calculated by including higher order MP, correlation energy corrections 1161, and therefore we co& dently believe that the MP2/6-31G* barrier height

N. Sanna et ai./J. Mot: Struct. 318 (1994) 217-235

is more reliable than the SCF and MP2/6-31G*// HF/6-31G** values. The drastic decrease in the height of the internal proton transfer barrier is readily explained by analysing the effects of electron correlation on the geometry of the C, and C,, symmetry structures. As is evident from Table 3, the MP2 geometries largely differ from the SCF ones. Firstly, electron correlation causes a conspicuous decrease of the 0. . .O distance from 2.525 A (HF/6-3lG*) to 2.507A (MP2/6-31G*) and a drastic shortening of the 0 .ss H hydrogen bond distance from 1.938 A (HF/6-31G*) to 1.807A (MP2/6-31G*). Thus, as observed for malonaldehyde [16], electron correlation shifts the proton towards the symmetric position found in the CZV symmetry transition state. As far as the saddle-point structure is concerned, the symmetrical intramolecular hydrogen bonding induces a closer approach of the oxygen atoms with respect to the asymmetric case, and electron correlation increases the 0 * . .O and 0. f . H distances from 2.252 and 1.204A (HF/63 lG*) to 2.320 and 1.243 A (MP2/6-31G*), respectively. Secondly, as for the hydrogen bond geometry, the troponoid ring structure is noticeably affected by electron correlation. The MP2 geometry of the ground state of tropolone results in significantly longer C=C bond distances (C2C3, C4C5 and C6C7) and in shorter C-C bond lengths (ClC2, C3C4, C5C6 and ClC7) than those obtained from the SCF calculations (see Table 3). Thus, as for the tropone molecule, the overall effect of electron correlation is to enhance rr-delocalization into the troponoid ring, and this is evident from the decrease in the alternation between single and double CC bond lengths (see Table 2). In contrast, the high K-conjugation of the C,, transition state ring emerging from the SCF calculations slightly decreases at the MP2 level. In fact, CC bond distances, substantially equivalent at the SCF level, show a small but appreciable bond fixation (C2C3 = 1.403 A, C3C4 = 1.389A and C4C5 = 1.397A) when electron correlation is included, Thus, we conclude that such structural changes regarding the intra-

225

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deform the molecular geometry of the monomer and, particularly, the bond angles subtending the C=O (C7ClC2) and OH (ClC2C3) groups. Similarly, changes are expected for the hydrogen bond geometry moving from monomer to dimer. In order to leave out the crystal field effects, the structural features of the ground state of tropolone have been derived in this paper from the comparison between ab initio geometries of tropone and tropolone molecules. From the SCF and MP2 computations (see Table 2), we observe that the replacement of one hydrogen atom with one OH group results in a significant increase of 7rdelocalization in the carbon ring. Such a tendency might be due to two synergic effects, that is, the presence of the OH substituent on the troponoid ring and the occurrence of intramolecular C=O . . . HO hydrogen bonding. The ab initio calculations allow us to investigate these effects separately. Firstly, the geometrical distortions of the troponoid ring caused by the OH substitution have been investigated by comparing the geometry of the non-hydrogen bonded rotamers of tropolone with that of the tropone molecule. Secondly, the structural consequences of intramolecular hydrogen bonding in tropolone have been taken into account by analysing the geometrical changes calculated on torsion of the

molecular hydrogen bond as well as the troponoid ring are consistent with the considerable drop of the proton exchange barrier height since the process of internal proton transfer involves an interchange between single and double bond character occurring in a range more limited at the MP2/63lG* (up to 0.055A) than at the HF/6-3lG** (0.107 A) levels. Table 3 shows that electron correlation brings the molecular geometry of tropolone into closer agreement with the X-ray structure [6]. In particular, the degree of CC bond fixation emerging from the MP2 geometry satisfactorily agrees with the X-ray determination and the r.m.s. deviation from the CC experimental value (0.200/a) is smaller than that calculated at the SCF level using the 3-21G (0.028%) [9] and the 6-3lG* (0.031%) [lo] basis sets. However, it can be seen immediately that the MP2 internal bond angles of the troponoid ring are not in good agreement with the experimental values and the differences, 1.4” (ClC2C3), 0.4 (C2C3C4), 0.7” (C3C4CS), 0.2” (C4C5C6), 0.7” (C5C6C7), 0.1” (C6C7Cl) and 0.9” (C7ClC2), are slightly smaller using the corresponding HF/631G* values (0.5, 0.4, 0.3, 0.4, 1.2, 0.2 and 0.8”). However, geometrical distortions are expected on the grounds of the fact that tropolone crystal consists of pairwise dimer molecules through C=O . . . HO intermolecular hydrogen bonding which may 0

$3 I

\

0

b/L/

OH

OH

AE =

-31 kJ/mol

( $ = 0”)

-11 kJ/mol

(*= 180”)

T -4 kJ/mol Fig. 3.

(4 = 90”)

Homodesmotic OH substitution reactions of tropone for cis (4 = O”), trans (I$ = 1807 and orthogonal orientations

of the OH bond.

(4 = 90”)

226

OH substituent from the cis (Fig. 1b) to tram (Fig. Id) rotamers. Geometrical d~tort~o~~due to the OH constitution The degree of CC bond fixation of the tropolone ring can be evaluated by forcing the OH substituent to take orientations preventing intramolecular hydrogen bonding interactions. The geometries of tropolone conformations resulting from a 180” (4) turn of the C-O bond, henceforth indicated as trans-tropolone (Fig. Id), and from a 90” (4) torsion of the C-O bond, indicated as orthogonaltropolone, have been optimized at the MP2/63lG* level and are reported in Table 3 along with the HF/6-3lG* results [lo]. When compared with the tropone geometry, the optimized structures of the trans- and o~hogonal-isomers of tropolone provide an indication of the deformation of the troponoid ring induced by the OH substitution in the absence (orthogonal-isomer) or in presence (transisomer) of C-O x-interactions. As suggested by the SCF and MP2 values of the differences between C=C and C-C bond lengths reported in Table 2, the degree of CC bond fixation of the seven-membered ring of tropone is substantially unaltered in the orthogonal-tropolone ring, whereas it decreases by approx. 0.01 A in the trans-rotamer. On these grounds, we may predict that the OH substitution of the troponoid ring enhances, although very slightly, 7r-delocalization, and consequently stabilizes the tropone sevenmembered ring exclusively for those orientations where the OH group is involved in n-interactions with the substituted carbon atom. In contrast, no stabilization of the tropone system results from the c-inductive withdrawing properties of the OH group. Further evidence of this is provided by the transfer energy, AE,, of the homodesmotic reactions [15] reported in Fig. 3, in which reactants and products contain equal numbers of CC and CO bonds between atoms of the same hybridization type. The reactions describe the transfer of the OH group from 1,4-pentadiene-3-one to tropone molecules in the absence of intramolecular hydrogen bonding (d, = 90 and 180’) and the AE, values

N. Sanna et al.~~. Mol. Struct. 318 (I994j 217-235

can be taken as an estimate of the stabilization energy resulting from the OH substitution of the tropone ring. The energy changes for reactions of Fig. 3 were estimated from the total energies of tropone and tropolone molecules and from the total energies of 1,4-pentadiene-3-one and 2hydroxy- 1,4-pentadiene-3-one determined from MP2/6-3 1G* geometry optimizations. The AE, values are very small but the negative sign indicates that the OH substitution slightly stabilizes the troponoid 7r-system. In addition, the conspicuous lengthening of the C-O bond calculated when the OH group is forced to lie perpendicularly to the ring plane, and the analysis of the Mulliken population reveal that the n-molecular orbitals of tropolone include prominent cont~butions from the oxygen atom of the OH group. The geometrical distortions from C,, symmetry of the tropone ring to C, symmetry of truns-tropolone are modest for the CC bond distances, but they are much more outstanding for the internal ring bond angles. For instance, the equivalent bond angles of the C,, symmetry tropone ring, ClC2C3 and C6C7Cl (130.4”), C2C3C4 and C5C6C7 (130.3”), C3C4C5 and C4C5C6 (128.1), are found to be significantly different in the C, symmetry tropolone ring (ClC2C3 = 129.5” and C6C7Cl = 132.5”, C2C3C4 = 130.8” and C5C6C7 = 128.9”, C3C4C5 = 129.9” and C4C5C6 = 128.9”). An interesting point is that the amount of geometrical distortion due to the OH substitution predicted at the SCF level [lo] compares well with the MP2 computations reported in this paper (see Table 2). Geometrical distortions due to OH torsion

Since a 180” turn of the C-O bond corresponds to a complete elimination of the internal hydrogen bonding of tropolone, the geometries of the transand cis-rotamers are helpful in investigating the structural consequences of intramolecular interaction. Considerable geometrical changes occur due to hydrogen bond formation with regard to the C=O and C-O bonds which lengthen by 0.014A (HF) and 0.017113(MP2) and shorten by

/0\

N. Sanna et al./J. Mol. Struct. 318 (1994) 217-235

221

H

0.009A (HF) and 0.017A (MP2), respectively. Intramolecular hydrogen bonding also causes an increase of 0.004 A (HF) and 0.023 A (MP2) of the OH bond length and a shortening by 0.016 A (HF) and 0.044A (MP2) of the 0 se+0 distance. On the grounds of these results it emerges that both the levels of calculation consistently describe the geometrical changes occurring on hydrogen bond formation, although the amount of such changes is more marked when electron correlation is considered. The torsion about the C-O bond from the open to the closed structures of tropolone also results in

‘0’

0

I

Fig. 4. Canonical forms for tropolone.

Table 4 Ab initio geometries of the O=C-C-O-H framework of 2-hydroxy-1,4-pentadiene-3-one and tropolone (bond lengths A and bond angles, deg) and relative stability, AE (kJ mol-‘), between the cis-, tram- and o~hogonal-isomers, 2-Hydroxy-1,4-pentadiene-3-one Cis HF/6-3X* c-c c-o c=o O-H o...o O...H H-O-C o-c-c o=c-c

1.509 1.344 1.202 0.952 2.536 1.967

Trans

1.513 1.346 1.193 0.947 2.525

Tropolone Orthogonala

1.511 1.364 1.195 0.948 2.590

Cisb

1.488 1.330 1.212 0.952 2.525 1.938

Transb

1.497 1.339 1.198 0.948 2.541

OrthogonalC

1.490 1.361 1.200 0.949 2.613

108.4 112.0 115.6

110.0 109.6 117.5

110.9 112.4 117.4

107.4 112.2 115.9

110.8 110.2 118.7

111.0 113.0 119.0

AE

0

31

39

0

43

51

MP2/6-31 G* c-c c-o c=o O-H o...o O...H

1.499 1.360 1.245 0.984 2.542 1.896

H-O-C o-c-c o=c-c AE

1.503 1.367 1.235 0.975 2.531

1.504 1.386 1.236 0.972 2.615

1.471 1.339 1.260 0.999 2.507 1.807

1.480 1.356 1.243 0.976 2.551

1.478 1.380 1.242 0.973 2.639

104.9 111.9 115.3

107.6 109.3 117.5

108.5 112.7 117.2

102.5 111.8 115.2

108.7 110.0 118.9

109.4 113.3 119.3

0

34

48

0

55

76

a The carbon skeleton was assumed to be planar whereas the oxygen atom of the OH group was found to lie 0.993 A (HF/631G*) and 0.148A (MP2/6-3lG*) above the carbon plane. b Ref. 10. ’ The carbon skeleton was assumed to be planar whereas the oxygen atom of the OH group was found to lie 0.067 A (HF/631G*) and 0.093 A (MP2/6-31G*) above the carbon pfane.

228

significant smoothing of the alternation between the C-C and C=C bond distances. With the exception of the C7Cl bond which shortens by about 0.02A on hydrogen bonding, both the single and double CC bonds lengthen and shorten, respectively, within about 0.01 A at the SCF [lo] and MP2 levels. Thus, intramolecular hydrogen bonding causes structural modifications regarding the entire molecule, consistent with increasing conjugation in the r-system. The structural modifications can be interpreted in terms of valence bond theory with the increase of the contribution of the polar canonical form shown in Fig. 4 to the electronic structure of tropolone. The polar structure makes the C=O group a stronger acceptor and the OH group a better proton donor and therefore its contribution should increase from the open structure to the intramolecularly hydrogen bonded tropolone. As for a wide number of species [12,13,22,23], hydrogen bonding is stengthened by the enhanced electron delocalization in the conjugated bond r-system [22,23]. The extra stability of the troponoid ring due to internal hydrogen bonding is also suggested by the AEr values of the homodesmotic reaction (4 = 0”) d’1spl a yed in Fig. 3. As for reactions previously described (4 = 180” and 4 = 90”), the transfer of the OH group from pentadienone to tropone stabilizes the seven-membered ring but the AEr value is larger in the presence of hydrogen bonding (4 = 0’) as a consequence of the fact that intramolecular interactions are favoured by delocalization in the conjugated bond system. This consideration is also supported by the additional computational results obtained for the 2hydroxy- 1,4-pentadiene-3-one molecule where the five-membered hydrogen bonding ring is 7r-conjugated with a five carbon open skeleton. HF/6-3 lG* and MP2/6-3 lG* geometry optimizations were carried out on the cis- (Fig. 2c) and trans- (Fig. 2e) isomers of 2-hydroxy-1,4-pentadiene-3-one with the aim of investigating the strength and geometry of the hydrogen bond and to compare them with those of tropolone. The results obtained are summarized as follows (see Table 4). Firstly, the energy difference between

N. Sanna et al./J. Mol. Strut. 318 (1994) 217-235

Fig. 5. C,, symmetry structure of tropolone

dimer.

the cis and trans-isomers, roughly taken as an estimate of the hydrogen bonding energy, is higher for tropolone (43 kJ mol-’ (HF) and 55 kJ mol-’ (MP2)) than for 2-hydroxy- 1,4-pentadiene-3-one (31 kJ mol-’ (HF) and 39 kJmol_’ (MP2)). Secondly, the 0. . . H and 0 +++0 hydrogen bond distances, shorter for tropolone than for 2-hydroxy1,4-pentadiene-3-one, indicate that the cyclic r-carbon skeleton of tropolone favours intramolecular interactions. The stabilization of the five-membered hydrogen bonding ring of tropolone is finally witnessed by the degree of structural changes occurring on a 180” turn of the C-O bond which is more marked than that calculated for the pentadienone derivative.

229

N. Sanna ei al.lJ. Mol. Struct. 318 (1994) 217-235 Table 5 Selected geometrical parameters Parameter

of tropolone

monomer and dimer (bond lengths, .J%and bond angles, deg) Dimer

Monomer HF/3-21Ga

I-IF/&31Gb

MP2/6-3 1G*

HF/3-21 G

HF/6-31G

1.246 1.343 2.616 2.723 2.226 1.795 0.967 1.468 1.355 1.420 1.354 1.422 1.353 1.437 115.6 117.0 118.3 128.4 130.6 129.4 126.7 130.2 131.0 123.7 0.0020 0.71

Bond lengths (A) Cl=0 1.234 cz-0 1.359 o...o 2.516 _ G...O’

1.243 1.359 2.550 -

1.260 1.339 2.507 _

O...H D...H’

1.930 _

2.03 1 -

1.807 _

OH ClC2 C2C3 c3c4 c4c5 C5C6 C6C7 C7Cl

0.974 1.476 1.337 1.425 1.343 1.428 1.341 1.439

0.958 1.470 1.346 1.427 1.351 1.429 1.350 1.441

0.999 1.471 1.381 1.406 1.381 1.412 1.376 1.436

1.237 1.341 2.601 2.655 2.181 1.703 0.984 1.472 1.347 1.417 1.347 1.420 1.344 1.436

116.1 113.0 111.6 129.9 129.6 129.1 127.3 130.3 130.3 123.5 0.0025 0.69

115.9 112.2 107.4 130.2 129.0 129.2 127.7 130.1 130.5 123.3 0.0020 0.75

115.0 116.9 116.9 129.3 129.8 129.6 127.1 130.6 130.4 123.4 0.0023 0.63

Bond angles (deg) C2Cl=O 115.6 ClC2-0 111.6 CZOH 108.5 ClC2C3 129.9 C2C3C4 129.7 c3c4c5 129.2 C4C5C6 127.2 C5C6C7 130.5 C6C7C 1 130.6 C7ClC2 122.9 ble 0.0028 SZf 0.83

a Ref. 9. b Ref. 10. ’ MP2/6-3lG* monomer geometry corrected for the HF/6-31G structural distortions d Ref. 6. e R.m.s. deviation from the expe~mental CC bond distances. ’ R.m.s. deviation from the experimental CCC bond angles.

Tr~po~one dimer Molecular geometry

On the basis of the previous considerations, it is interesting to note that the geometrical distortions of the tropolone ring induced by intramolecular interactions are comparable both at the SCF and MP2 levels. With the assurance that hydrogen

MP2/6-31G* correctedC

1.263 1.323 2.573 2.002 1.008 1.469 1.390 1.399 1.384 1.405 1.379 1.432

128.7 130.0 129.5 127.1 130.0 131.2 123.5 0.0020 0.52

X-rayd

1.26113) 1.333(3) 2.553 2.754 1.98 1.98 0.94(3) 1.454(4) 1.379(4) 1.393(4) 1.341(4) 1.410(4) 1.373(4) 1.410(3) 115.4(2) 114.7(Z) 107 128.8(3) 129.4(3) 129.9(3) 127.5(3) 129.4(3) 130.6(2) 124.2(Z)

induced by dimerization.

bonding structural modifications may be quahtatively predicted from SCF calculations, we have focused our attention on the geometrical perturbation caused by dimerization. The ~ntrosymmetrical dimer of crystal tropolone, where the molecules are hydrogen bonded to each other through two C=O.. . HO bonds, has been reproduced by the CZr, symmetry struc-

3379.3 3375.8 3366.7 3348.5 3336.0 3330.1 1879.9 1845.9 1795.6 1751.5 1648.4 1594.6 1592.1 1478.1 1419.4 1377.8

1360.1

B2

HF/6-31 G”

Al B2 Al B2 Al B2 Al B2 Al Al B2 Al B2 B2 Al Al

Tropone

Table 6. Ab initio and experimental

CCH/C-C

CH CH CH CH CH CH c=c/c=o c=c c=o/c=c c=c CCH CCH CCH CCH CCH CCH

3377.9 3374.9 3367.5 3350.6 3337.7 3332.2 1944.6 1852.2 1866.3 1759.5 1633.7 1579.9 1568.3 1422.7 1394.5 1358.9

1343.5

Major internal coordinates”

Tropolone Monomer HF/4-3lG* 3943.3 3380.1 3374.4 3365.5 3347.7 3338.0 1866.1 1836.0 1895.5 1670.4 1651.9 1575.5 1594.4 1453.0 1387.9 1420.2 1356.7 1315.6

Monomerb HF,‘6-31G 3922.9 3383.9 3380.7 3369.6 3348.6 3337.6 1864.4 1850.5 1751.9 1676.8 1655.4 1594.5 1591.0 1461.3 1421.6 1379.6 1365.8 1307.8

1379.7 1346.3 1316.5

1844.0 1839.3 1741.1 1648.5 1660.6 1596.1 1584.1 1466.7 1415.2

3741.5 3384.3 3380.7 3369.3 3345.9 3335.8

DimerC HF/6-31G

frequencies (cm-‘) of tropone and tropolone molecules

HF/4-3 1G*

vibrational

1.09 1.08 0.96

0.88 0.88 0.89 0.88 0.88 0.89 0.96 0.97 0.90

0.90 0.90 0.90 0.90

1499 1481 1252

1635 1628 1565 1470 1460 1427 1522 1412 1274

3121 3055 3055 3030 3023 3006

0.80 0.90

Monomerb exp

Scale factorb

Solidd

1232

1480

1412 1265

1421

1613 1603 1546 1465

exp

CCH/COH CCH/COH C-O/CCH/CC

C=C/COH,‘CCH C=C/C-C/CCH C=O/C=C/CCH CC/CCH CCH CCH CCH/COH/CC CCH/COH/CC CCH

OH CH CH CH CH CH

Major internal coordinatesb

1210.0 1195.3 1178.9 1024.4 934.8 903.4 663.2 651.2 378.9 305.5 83.0

A2 Bl A2 Bl A2 81 Bl A2 Bl A2 Bl CH twisting CH wagging CH twisting CH wag~ng/skel CH twisting CH waggingiskel CH wagging/skel skel skel/C=O wagging skel skel

CCH/C-C/skel C-C/CCH C-C/skel c-c skel/C-C skel/C=O skel skei

1143.7 1037.1 982.4 818.7 728.9 583.2 485.5 406.5 391.7 1202.7 1181.8 1087.3 988.3 886.7 797.3 666.9 644.9 449.1 399.6 212.3 106.3

(A,) IR active bands of the tropolone

1171.7 1161.7 1144.5 1006.0 913.7 891.6 652.5 627.6 359.9 296.0 71.0

1096.1 1004.6 944.2 900.2 759.5 540.5 437.4 415.0

’ Ref. 8. b Ref. 10; exp = experimental. ’ In-plane (B,) and out-of-plane d This work; exp = experimental.

Ii 10.3 1016.6 977.1 915.5 772.0 542.8 446.8 427.3

B2 Al B2 Al Al B2 B2 Al

dimer species.

1136.8 1029.7 950.5 808.0 724.3 588.8 480.9 405.0 388.5 1162.4 1141.1 1040.3 964.1 871.5 789.0 630.1 667.5 450.2 380.2 203.3 98.5

1151.5 1049.1 984.8 825.9 732.2 581.7 487.6 423.4 373.9 1200.8 1181.0 1084.4 991.7 882.6 799.2 668.3 871.2 449.8 406.0 212.3 105.6 1.00 0.92 0.89 0.91 0.93 0.94 0.89 0.89 0.89 0.83 0.83 0.84 0.84 0.87 0.85 1.13 1.12 0.75 0.68 0.82 1.03

1146 952 875 742 675 551 434 359 349 1000 983 908 828 774 676 751 720 33s 272 177 110 790 672 753 860

1005 985 918

436

957 874 753 672

~~/CC~/COH CC/CCH CC/CCH/skel skel skel/CC skel skel/CCO skel/CCO CCO/skel CH wagging CH wagging CH wagging CH wagging CH/skel CH/skel skel/COH torsion COH torsion skel/COH wagging skel skel/COH wagging skel/OCCO twisting

232

N. Sanna et al./J. Mol. Struct. 318 (1994) 217-235

0

1 sbo

1 sbo

14bo

WRV~NUME~RS

CM-1

Fig. 6. FT-IR spectra in the 1700-l IOOcn-’ region of dilute (a), concentrated

ture shown in Fig. 5. Geometry optimizations were carried out at the HF/3-21G and HF/6-3 1G levels and the optimized structure is compared with the analogous monomer geometries [9,10] in Table 5. As clearly emerges from the computational results, dimerization results in appreciable structural changes in the groups involved in the intermolecular hydrogen bonding as well as in the entire sevenmembered ring. The occurrence of the C=O ’ . ’ HO bonding leads to a considerable increase, by 0.09 A, of the intramolecular 0 . . .O separation (2.616 A) which compares well with the intermolecular 0. . .O distance (2.723 A). However, the He .vO intramolecular bond length (2.226 A) is considerably longer than the intermolecular 0 ... H value (1.795A). On the basis of SCF calculations, we may conclude that the H atom participates in a bifurcated hydrogen bond where intermolecular interaction superimposes and weakens the intramolecular hydrogen bonding occurring in the tropolone monomer. Consequently, dimerization

z 3bo

I zbo

1’

30

(b), and annealed (c), samples in an N2 matrix.

originates a lengthening of the OH and C=O bonds and a shortening of the C-O and Cl-C2 bonds, indicating an appreciable increase of x-conjugation in the 0-Cl-C2=0 system. This is reflected through the n-system of the carbon atoms since the formal CC double bonds of the tropolone dimer are longer and the single CC bonds are shorter with respect to the values for the monomer, However, since the SCF level underestimates rr-delocalization through the carbon skeleton, it is likely that dimerization geometrical perturbations are more marked than those determined from the present calculations. Notwithstanding this, we may conclude that both intramolecular and intermolecular hydrogen bonding causes structural modifications arising from the contribution of the polar canonical form of Fig. 4 as resonance structure to the ground state electronic structure of tropolone. Consistent with CO and C--O polarization, dimerization also results in a large decrease (1.5”) of the bond angle subtend-

233

N. Sanna et al./J. Mol. Struct. 318 (1994) 217-235

ing the OH-substituted carbon atom and in a small increase (0.2”) of that subtending the carbonyl group. We can therefore conclude that the main cause of the differences observed between the MP2/6-31G* and X-ray structures is probably due to the intermolecular interactions occurring in the solid phase. This follows from the observation that the MP2/631G* structure, corrected for the crystal field effect estimated from HF/&3 1G calculations, significantly improves the agreement with the X-ray geometry (see Table 5). Vibrational frequencies

The geometrical consequences of dimerization predicted in this paper by ab initio calculations are plausible in view of a wide number of systems where hydrogen bonding is assisted by resonance structures ]22,23]. Alternatively, the structural perturbations due to hydrogen bonding can be investigated by a combined study of the experimental and SCF vibrational spectra of the tropolone monomer and dimer. The available spectroscopic observations on the tropolone monomer were accurately investigated in a recent paper with the support of HF/631G vibrational analysis [IO]. The IR spectrum of the monomeric species, previously measured in a Ne matrix [7l, has now been reproduced in isolated, concentrated and annealed Ar and N2 matrices with the aim of gaining info~ation on the effect of the intera~ions between the trapped molecule and the matrix and on the effect that the different deposition conditions have on the IR spectrum. The assigned vibrational spectrum of the monomer serves as a reference to study the FT-IR spectrum of a thin, solid tropolone film where the molecules are self-associated by hydrogen bonds. As can be seen from Table 6, some bands of the matrix spectra undergo appreciable frequency shifts in the solid spectra and ab initio calculations were employed to explain them. The spectroscopic features of the tropone ring were further investigated in this paper by HF/431G* frequency calculations on the tropone and tropolone molecules. As can be seen from Table

6, including pola~zation

functions results in small frequency differences, the most remarkable of which occur for the normal modes with a prominent C=O stretching component which, as usual for polarized functions [15], are largely overestimated. Concerning the out-of-plane modes, the HF/4-31G* level reproduces the experimental frequencies with better agreement with respect to the HF/6-3 1G values [8,10]. An interesting point is that the planarity of the tropone ring, emerging from HF/6-31G computations [8,10], is also confirmed by including polarized basis functions, although the frequency value of the out-of-plane deformation mode is once again very low for tropone (71 cm-‘) and tropolone (98 cm-‘) molecules. Notwithstanding such differences, the normal mode analysis suggests that the vibrational frequencies and the normal coordinates are substantially inde~ndent of the choice of basis set. With this assurance and in the absence of ab initio frequency calculations at the highest levels of the basis set and electron correlations, which the size of tropolone dimer renders not feasible, we might transfer the scale factors LQ,~/LQ~ (see Table 6) from the HF/6-31G monomeric spectrum [IO] to an HF/6-31G dimeric one and interpret the frequency shifts observed from the monomeric to the solid state spectra. This procedure can be easily followed for the dimer normal modes which readily correlate with the corresponding normal modes of the monomer, such as the CH, OH and ring stretching vibrations. Despite this fact, some normal coordinates, such as those substantial contributions from incorporating in-plane COH bending and from C-O stretching internal coordinates, or those involving in-plane ring defo~ations, are substantially modified on dimerization and therefore it is not obvious to correlate the corresponding monomer and dimer normal modes. Therefore, the wavenumber shifts observed for these vibrations are not easily related to the structural modifications due to self-association. The conclusions derived from the theoretical spectroscopic investigations are summarized in Table 6 and provide further proof that inter-

234

molelcular hydrogen bonding increases the extent of 7r-conjugation onto the tropone ring. Ab initio calculations indicate that dimerization causes considerable frequency shifts of the C=O, OH and CC stretching modes. In particular, the C=O and OH stretching frequencies are lowered, whereas the normal modes, with a prevalent contribution of CC stretching, undergo frequency shifts in line with the ring structural modification. The variation of the CC bond fixation degree calculated on dimerization is entirely reflected in the C=C and C-C stretching frequencies, which respectively decrease and increase as a result of intermolecular association. A conspicuous change also occurs to the out-of-plane OH bending mode, the frequency of which increases by about 230 cm-r owing to the fact that intermolecular hydrogen bonding stiffens the COH torsion. As an alternative approach to the study of the spectral changes induced by intermolecular hydrogen bonding, the FT-IR spectra of tropolone were measured in con~ntrated and annealed Ar and Nz matrices in an effort to encourage association of individual monomeric species. Self-association processes in a matrix are indeed quite common, although rather limited, for molecules able to form intermolecular hydrogen bonding [24]. In spite of the fact that the region ranging from 1200 to 400 cm-’ is not dependent on the deposition conditions, the spectra displayed in Fig. 6 indicate that concentrated (Fig. 6b) and annealed (Fig. 6c) matrices exhibit spectral features differing from those of the isolated samples (Fig. 6a). There is no strai~tfo~ard explanation of the spectral changes occurring in this region since several factors could contribute to modify the band shape of the corresponding vibrations. We should bear in mind that proton tunnelling splits all the bands relative to the normal modes involving heavy atom skeletal changes a~ompanying the proton exchange process. In particular, the C=O, C-O and C=C mixed stretching modes exhibit a rather complex pattern because of proton tunnelling. The relative intensity of the components of each multiplet shows outstanding changes with the deposition

N. Sanna et a1.j.l. Mol. Struct. 318 (1994) 217-235

conditions. Attributing the origin of the multiplets and their changes to matrix effects seem unlikely since the pattern and the splitting of each multiplet is comparable in Ne 171,Ar and N2 matrices. A plausible explanation of the dependence of the FT-IR spectra on the matrix concentration and on the vaporization temperature is found in the presence of a limited but observable amount of aggregates. The dimer absorptions, consistent with the ab initio predictions, would overlap the lower frequency components of the monomer multiple& and consequently lead to changes in the band shapes in this region. Such an interpretation finds support in the fact that the concentration dependent peaks occur very closely in frequency to the bands measured in the spectrum of tropolone solid samples (see Table 6). Conclusions The most interesting conclusions made from this work are summa~zed as follows. (1) Ab initio calculations including electron correlation corrections indicate that the troponoid ring shows a pronounced alternation between typical single and double CC bonds. Such a conclusion is argued from MP2/6-31G* geometry optimizations of tropone and tropolone systems. Comparisons with the experimental crystal structures attest the quality of the MP2 geometries and reveal that the SCF determinations overestimate the CC degree of bond fixation. (2) In the absence of accurate molecular geometries of the free tropone and tropolone molecules, intrinsic structural features of the troponoid system were obtained in this paper from MP2/6-31G* geometry optimizations of the internal rotation isomers of tropolone. The structural changes occurring on 180” and 90” torsions of the OH group and the results of a series of hom~esmotic reactions indicate that the n-delocalization in the troponoid system is slightly enhanced by replacing a hydrogen atom with an OH group and is further increased by C=O . +. HO intramolecular hydrogen bond formation. The comparison with the structural and

235

N. Sanna et al.1.l. Mof. Struct. 318 (1994) 217-235

energetic features of the analogous systems 1,4pentadiene-3-one and 2-hydroxy-l,~penta~ene-3one, where the five-membered hydrogen bonding ring forms part of an open carbon skeleton, indicates that intramolecular hydrogen bonding in tropolone is favoured by electron delocalization through the r-system of the troponoid ring. (3) As reported for some intramolecularly hydrogen bonded species such as malonaldehyde, the MP2/6-3lG* level provides a more reliable description of intramolecular interaction than the SCF calculations. Second-order perturbation theory reduces the asymmetry of hydrogen bonds of tropolone and, enhancing 7r-delocalization in the carbon ring, narrows the range of the interchange between single and double CC bond character occurring in the proton tunnelling. ‘Both the effects result in decreasing the energy separation between the C, and C,, symmetry structures, passing from 70 kJ mol-’ (HF/6-31G*) to the more reliable value 25.8 kJmol_’ (MP2/6-3lG*), which is closer to the experimental barrier height of malonaldehyde (25-28 kJ mol-‘). (4) Crystal field effects on the molecular structure and vibrational spectrum of tropolone have been estimated from HF/6-31G calculations on the Czi, symmetry structure of the tropolone dimer. Selfassociation induces geometry perturbations and vibrational frequency shifts consistent with the increasing conjugation through the entire n-system of the molecule, FT-IR spectra of the tropolone monomer isolated in Ar and N2 cryogenic matrices are compared with the FT-IR spectra of solid samples, and the observed frequency shifts reasonably agree with the ab initio predictions. Acknowledgements

The authors acknowledge MURST for financial support and the CASPUR Project for providing computer time. References 1 M.J. Barrow, OS. Mills and G. Filippini, Sot., Chem. Commun., (1973) 66.

J. Chem.

2 M. Ogasawara, T. Iijima and M. Kimura, Bull. Chem. Sot. Jpn., 45 (1972) 3277. 3 R.A. Creswell, J. Mol. Spectrosc., 51 (1974) 111. 4 J.W. Emsley and J.C. Lindon, Mol. Phys., 25 (1973) 541. 5 M. Kimura and M. Kubo, Bull. Chem. Sot. Jpn., 26 (1953) 250. 6 H. Shimanouchi and Y. Sasada, Acta Crystallogr. Sect B, 29 (1973) 81. 7 R.L. Redington and T.E. Redington, J. Mol. Spectrosc., 78 (1979) 229, and references cited therein. 8 R.L. Redington, !%A. Latimer and C.W. Bock, J. Am. Chem. Sot., 94 (1990) 163. 9 M.A. Rios and J. Rodriguez, Can, J. Chem., 69 (1990) 201. 10 R.L. R~ington and C.W. Bock, J. Phys. Chem., 95 (1991) 10285. 11 G.A. Jeffrey and W. Saenger, Hydrogen Bonding in Biological Structures, Springer-Verlag, Berlin, 1991. 12 F. Ramondo, L. Bencivenni, G. Portalone and A. Domenicano, Struct. Chem., in press. 13 F. Ramondo, S. Nunziante Cesaro and L. Bencivenni, J. Mol. Struct., 291 (1993) 219. 14 L. Bencivenni, A. Domenicano, G. Portalone and F. Ramondo, Abstracts, 21st Meeting of Italian Crystallographic Association, Parma, l-3 October 1991, Parma, 1991, p. 121. 15 W.J. Hehre, L. Radom, P.v.R. Schleyer and J.A. Pople, Ab Initio Molecular Orbital Theory, Wiley, New York, 1986. 16 M.J. Frisch, A.C. Scheiner, H.F. Schaefer, III and J.S. Binkley, J. Chem. Phys., 82 (1985) 4194. 17 C. Msller and MS. Plesset, Phys. Rev., 46 (1934) 618. 18 R. Poirer, R. Kari and LG. Csizmadia, Handbook of Gaussian Basis Sets, Elsevier, Amsterdam 198.5. 19 M.J. Frisch, M. Head-Gordon and J.A. Pople, Chem. Phys. Lett., 166 (1990) 28 1, and references cited therein. 20 M.J. Frisch, M. Head-Gordon, G.W. Trucks, J.B. Foresman, H.B. Schlegel, K. Raghavachari, M. Robb, J.S. Binkley, C. Gonzalez, D.J. DeFrees, D.J. Fox, R.A. Whiteside, R. Seeger, CF. Melius, J. Baker, R.L. Martin, L.R. Kahn, J.J.P. Stewart, S. Topiol and J.A. Pople, GAUSSIAN 90,Gaussian Inc., Pittsburgh, PA, 1990. 21 T. Carrington and W.H. Miller, J. Chem. Phys., 84 (1986) 4364. 22 G. Gilli and V. Bertolasi, in 2. Rappoport (Ed.), The Chemistry of Enols, Wiley, New York, 1990, p. 713. 23 V. Bertolasi, P. Gilli, V. Ferretti and G. Gilli, J. Am. Chem. Sot., 113 (1991) 4917. 24 M. Szczesniak, M.J. Nowak, K. Szczepaniak and W.B. Person, Spectrochim. Acta, Part A, 41 (1985) 237.