3716
J. Phys. Chem. 1993,97, 3716-3721
Quinonic Tautomer Present in a Crystal of N-(2,3-Dihydroxybenzylidene)isopropylamine: Structural and Vibrational Data and AM1 Calculations Micheline Caries,' FrMderica Mansilla-Koblavi,z Jules Abodou Tenon,* Thomas Yao N'Guessan,t and Hubert Bodot'J Laboratoire de Physique des Interactions Ioniques et Molbculaires, URA CNRS 773, Universitb de Provence, Centre de Saint-Jbrame, Case 542, 13397 Marseille Cedex 20, France, and D6partements de Physique et de Chimie, Universitb Nationale de Cate d'lvoire, 22 BP 582, Abidjan 22, Cate d'lvoire Received: November 17, 1992
The experimental structural data of the crystalline title compound are compared with geometrical parameters calculated for the phenolic (P) and quinonic (Q) tautomers; evidences are provided for an equilibrium largely shifted toward the Q tautomer. This conclusion is also supported by the IR spectral analysis. Involved in intraand in intermolecular hydrogen bonds, the OH group located in position 3 appears as a probe of the tautomeric equilibrium. The corresponding stretching IR absorption band presents several components; the more intense one is assigned to QQ dimers and others to P Q and PP dimers which are also present in the crystal. Two other components of that broad IR absorption band belong to Q Q and PQ N-H stretching vibrations. To account for qualitative informations about the tautomeric equilibrium (P dominates, but Q is present in C C 4 solution) and about the salicylidenamine derivatives (Q is a minor component in crystal and is absent in CC14solution), semiempirical AM 1 calculations were performed on the methylamine derivatives; the H.-0 hydrogen bond involved in the five-membered pseudocycle appears stronger in Q tautomers. Intermolecular hydrogen bonds contribute to increase the Q tautomer stability as proved by its dominance in H20 solution.
Introduction Previous investigations of a series of salicylidenaminesC6H4(2-OH)-CH=N-R (1P tautomer) provided new spectroscopic, photochemical, and kinetic data.l.2 Our own results revealed that quinonic tautomers (also considered as zwitterionic forms3) C6H4(2-O)=cH-NH-R (1Qtautomer) were present as minor components, even in the photochromic crystals before photoisomerization.2 Among these results, we must emphasize our observationson the crystalline film of a derivative (R = (CH3)2CH-) which is thermochromic at room temperature and behave as a photochromic species when cooled down to -30 "C. Unfortunatly, that derivative is inappropriate to perform a crystallographic study owing to its low melting point. For compounds with the same R group, but having a second hydroxyl group in the ortho position with respect to the first one, we expected melting points being higher than the salicylidenamine ones. Moreover, this second hydroxyl group might be, through its 0-H vibrational frequency, an efficient probe for the P and Q tautomeric states:
2P
20
Five derivatives of the 2 series (R = C ~ H Sp-CH3-CbH4, , o-CIC6H4,(CH3)2CH, and cyclopropyl) have been synthesized; their crystallographic and molecular structures have been determined (X-ray diffraction) and the corresponding data will be concomitantly p~blished.~ Some bond lengths which are good tautomeric probes indicate that the titlecompound 2 (R = (CH&CH) predominantly appears as Q tautomer. Therefore, that derivative has been selected in order to confirm this observation through a FT-IR study of the
' Universitt de Provence. 1
UniversitC Nationale de Cdte d'lvoire.
0022-3654f 9312097-3716S04.00f 0
crystal at different temperatures, and also in an apolar solvent; UV-visible spectra are also reported. In this paper, the relative Q tautomer stabilization will be investigated through semiempirical AM1 calculations of P and Q tautomers of 1 (R = C H d and of 2 (R = CHI). Moreover the relative energies of QQ and PPdimers will becalculated using thesame semiempiricalmethod.
Experimental Section The synthesis of N-( 2,3-dihydroxybenzylidene)isopropylamine has beeneasilyperformed by heating up toreflux for a few minutes an equimolar mixture of 2,3-dihydroxybenzaldehyde(Aldrich) and isopropylamine in ethanol solution. The recrystallization has been done in ethanol; mp 120-121 "C. The IR spectra were obtained from a 7199 Nicolet FT-IR spectrometer at 2-cm-' resolution. For the CC14 solution, the concentration was 0.1 mol L-]and the cell was equipped with KBr windows separated by 100 pm. For the crystalline sample, it was studied as KBr pellets (1 wt %) in a Specac cryostat. Decompositions of broad absorption bands were performed, each component being considered as 100%Lorentzian; the half-height widths were around 63 and 33 cm-1 for X-H vibrations (X = 0 or N) respectively at 300 or 80 K; for C-Y vibrations, the corresponding values are 20 and 17 cm-1 (Y = 0 or N). Minor components may be added to improve the fitting of the absorption bands around their wings, but it is meaningless. The electronic absorption spectra were recorded on a Model 25 Beckman spectrometer.
Basic Structural Data In Table I, we report the experimental and calculated vahes for the main structural parameters. Within the 2 series, the experiment'al bond lengths and valence angles are closer to the 2Q calculated values than to the 2P ones; only three exceptions are observed over 14 presented data. The most significant examples are those where large differences are forecasted between 2P and 24 data: C(2)-0(2), C( l)-C(7), C(l)-C(2)-C(3), and C(2)-C( 3)-O( 3). 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 3717
N-(2,3-Dihydroxybenzylidene)isopropylamineCrystal
TABLE I: Selected Bond Lengths (A)and Angles (deg): Experimental' and Cal~ulated4~ Values bond or angle' C(l)-C(2) w-c(3) C(3)-C(4) C(4)-C(5) C(5)-C(6) C(a)-C(l) C(2)-0(2) C(3)-0(3) c (1 ~ 7 ) C(7)-N C(l)-C(2)-C(3) C(2)-C(3)-C(4) C(l)-C(2)-0(2) C(2)-C(3)-0(3)
2(R= (CH3)lCH) exp4 1.433(3) 1.430(3) 1.370(3) 1.409(3) 1.355(3) 1.425(3) 1.294(2) 1.372(2) 1.412(3) 1.301(3) 116.6(2) 121.4(2) 123.0(2) 118.6(2)
2P (R CH3) calcb
2Q (R CH3) calcb
1.403 1.422 1.395 1.397 1.387 1.409 1.372 1.375 1.468 1.287 120.1 120.3 126.2 122.3
1.452 1.475 1.361 1.432 1.356 1.442 1.255 1.376 1.389 1.346 116.6 122.4 124.3 118.4
C ~ HI I) ~ 1P (R = CH3) exp* calcb
1 (R
1Q (R = CH3)
1P (R CHJ Calct
1Q (R = CHj)
calcb
Cab
1.377(8) 1.386(9) 1.397(9) 1.341(10) 1.376(9) 1.41a(8) 1.369(7)
1.408 1.414 1.384 1.401 1.386 1.408 1.367
1.397 1.390 1.375 1.392 1.375 1.392 1.351
1.462 1.466 1.351 1.436 1.387 1.440 1.252
1.449 1.449 1.343 1.435 1.344 1.430 1.253
i.448(8) 1.284(7) 121.6(6) 118.2(7) 120.9(6)
1.467 1.287 120.2 120.1 125.7
1.464 1.262 119.13 120.50 122.31
1.389 1.347 116.5 121.9 122.9
1.382 1.313 115.24 121.81 122.53
References 4 and 8. This work. Numbering as in the formula. R = 2,4,6-trimethylphenyl. e Reference Sc. m
'
P
o?dOO
3bOO
3 h O
UAVENuMBER
2600
2400 UAVENUMBER
Ln
0. 0
0 -
Figure 1. Molecular packing in the crystal of 2 (R = (CH3)2CH). For the 1 series, the best agreements occur between experimental values (that of R = 2,4,6-trimethyl~henyl)~and 1P calculated ones; they are often outside the P-Q range, but close to P values. The only important disagreement is that of C( 1)C(2)-0(2). As the previous studies2 have proven that the P tautomer is dominant in the crystal of 1 (R = 2,4,6-trimethylphenyl), the comparison between experimental and calculated geometrical parameters appears as an efficient method to identify the major tautomer. Obviously, the Q tautomer is the most abundant one in the crystal of 2 (R = (CH3)zCH). Additional structural details will be reported further with emphasis on those related to the intramolecular hydrogen bonds. Nevertheless,for the crystal of 2 (R = (CH&CH), a preliminary view on the packing must be shown (Figure 1); intermolecular hydrogen bonds associate two molecules (I and 11) connected by a center of symmetry through a IO-membered pseudocycle:
O(2)-C( 2)-C( 3)-0( 3)-H( 3)'*-0( 2)-C( 2)-C( 3)O(3)-H( 3)"-0(2)' In such dimers, the intermolecular 0(2)-0(3) distance is very short (2.70 A). The salient feature of the pseudocycle chair conformation is the distance (0.87 A) between the two molecular planes.
Spectroscopic Data of 2 (R = (CH3)zCH) CLH and N-H Stretc- Vibrations. Owing to their sensitivity to hydrogen bonding, the 0-H stretching vibrations have to be
Ln h
9-
Figure 2. Infrared spectra of 2 (R = (CH&CH): (a, top) a t 140 K (crystal); (b, bottom) in CC14 solution a t 300 K. examined first. We must remind that the 0(2)-H(2) stretching modes of P tautomer in the 1 series give very broad band with low intensity in the 2500-2800-~m-~range;2 such a large shift arises from the strong 0-H-N hydrogen bond occurring in a six-membered pseudocycle.2,9 In the crystal, the 2P tautomer concentration is too low to give a detectable IR absorption band for v(0(2)-H(2)). On the other hand, the 0(3)-H(3) and N-H(3) stretching vibrations display, in the crystalline state, a broad IR absorption band ranging from 3 100 to 3400 cm-1 (Figure 2a) analyzed with five components, the strongest of which is located at 3212 cm-i (Table 11).
3718 The Journal of Physical Chemistry, Vol. 97, No. 15, 1993
Carles et al.
TABLE 11: IR Absorption Bands and Components (Fr uencies in cm-I), Relative Integrated Absorbances ( A Z A ) , and Tentative Assignments for X-H and C=Y Stretching Vibrations of 2 (R = (CH3)2CH)' Y (300 K) A/xA Y (80 K) A/zA assignments 3212
0.55
LA (cm- I)
34.8
1610 1630 1642 1652 1670 LA (cm-I)
0.02 0.14 0.67 0.15 0.01 26.5
3179
0.71
0-H/QQ
42.4 1612 1627 1645 1659 1677
0.04 0.07 0.81 0.07 0.01 31.5
\ \
C=N/PQ? C=N/PP C=O (antisym)/QQ C=O (sym)/QQ C=O/PQ
CCls solution (300 K): 3260 (N-H/Q); 3400 (O-H/Q); 3548 (0H/P); 1600 and 1616 (C-C, arom); 1635 (C=N/P).
With the deuterated molecule, these absorption bands shift to frequencies around 2400 cm-' (v(O-H)/v(O-D) ratio = 1.34). The corresponding bands display a similar distribution of components which suggest that harmonic or combination bands do not significantly contribute to the absorption. When the molecules are transferred from the crystalline state to the CC14solution, the spectrum is appreciably modified;dilution does not change the spectrum (Figure 2). We observe (a) a very large frequency shift (more than 300 cm-I) and (b) an IR absorption band at 3548 cm-I, the sharpness of which confirms that only monomers are present in solution; two very weak bands at 3400 and 3260 cm-I (Figure 2b) corresponding to the other tautomer. The proximity of 3548 cm-1 with 3567 cm-I corresponding to v(0-H) of 1,2-dihydroxybenzene allow us to identify P as the more abundant tautomer in apolar solution. As with 1P,2 the 0(2)-H(2) stretching absorption band is observed in the 2500-2800-cm-I range, but it is a very broad one with low intensity, which suggests some couplings such as those observed precedently. For the other absorption bands observed at 3400 and 3260 cm-I, despite their low intensities, the assignment to the minor Q tautomer seems obvious; the 3400-cm-I absorption bands are the closest ones with respect to the hydrogen-bonded 0-H frequency (3481 cm-l)lo of 3-hydroxy-2-butanone, and 3260 cm-l is attributable to v(N-H). To explain these results, we have to consider the disappearance of intermolecular hydrogen bonds which stabilize the QQ dimer compared to PP one in the crystal. As more than three IR absorption components are observed for the molecules in the crystal, we cannot limit our hypothesis to a crystallographic disorder resulting from the only presence of PP and QQ homogeneous dimers. We must consider that PQ heterogeneous dimers are also present. The most intense component located at 3212 cm-I (300 K) may be assigned to the asymmetric v(0(3)-H(3)) of QQ, Its relative integrated absorbance get a 29% increase" when temperature is lowered down to 80 K (Table 11); simultaneously, the relativeintegrated intensities oftheothercomponentsdecrease or does not change much. Important frequency shiftsareobserved. Owing to our assignments done for the CC14solution spectrum, the asymmetric v(N-H) of QQ dimers is certainly located at a frequency lower than that of the major component; in other respects, the asymmetric v(0-H) of PP dimers is certainly at higher frequencies. Such tentative assignments are no more possible with PQ dimers, but the presence of other components gives evidence for absorptions assignable to one v(N-H) and two v(0-H) modes of PQ dimers (Table 11). C 4 and C-N Stretching Vibrations. Crystalline 2 (R = (CH3)2CH) presents a broad IR absorption band centered a t 1642cm-I, the analysis of which gives several components (Figure 3 and Table 11). Owing to its increase when temperature is lowered
Figure 3. The 170&1595-~m-~ absorption band of 2 (R = (CH3)2CH) at 140 K (crystal) and its components.
down to 80 K, the 1642 cm-' (300 K) component is assigned to the antisymmetric v(C=O) mode of QQ dimer. The 1630-cm--' component shows an opposite temperature behavior and may be attributed to the asymmetric v(C=N) mode of the PP dimer; the frequency is close to that of 1 (R = (CH3)2CH) (1631 cm-1).2 Two other components have to be assigned to v ( C 4 ) and v(C=N) of PQ dimers; the component at 1652 cm-I might be that of the symmetric v(C==O) mode of QQ, not totally forbidden owing to crystal defaults removing the centrosymmetry. In the same frequency range, the IR spectrum of the CC14 solution presents a sharp intense absorption band at 1635 cm-1, corresponding to the v(C=N) mode of P tautomer (cf. 1634cm-1 for 1PIs2Minor neighboring absorption bands are detected at 1652,1616,and 1600cm-I. Thelast twoonescorrespond toC-C stretching mode of a benzene cycle (8a and 8b in Wilson's notation).12 The 1652-cm-I absorption band may be attributed to thev(C=O) modeoftheQ tautomer which is thelessabundant one in CC14 solution. Other Vibrations. The experimental frequencies reported in Table 111(1600450-cm-I range) correspond to Q and P tautomers which are dominant in the crystal and in the solution respectively. Therefore, it is easy to select the absorption bands belonging to each tautomer. The Q absorption bands are easily identified with well-known group frequenciesI3 associated with the following: (a) the keto enamine group, the vibrational modes of which are very similar to that of an amide group: 1550 and 1520 (type 11); 1240-1220 (type 111); 902 and 721 (./(N-H)); 580 and 565 cm-I (type IV). (b) The quinoid group with its in-plane and out-of-plane C-H deformations which are located close to aromatic cycle ones: 1130 and 1125; 1030 and 1020; 755 and 702 cm-1. Some IR absorption bands are present in the two spectra at the same frequencies and with similar intensities; they are attributed to isopropyl group vibrations. The specific P IR absorption bands of 2 (R = (CH,)2CH) in Ccl4 solution have frequencies very close to those of 1 (R = (CH3)2CH);*therefore, their assignment to group frequencies is straightforward. In that solution spectrum, other absorption bands are clearly out of the P absorption ranges; they have frequencies corresponding ( A10 cm-I) to intense absorption bands observed with the crystal and they indisputably belong to Q tautomer (mainly 1560, 1540,904, and 760 cm-I). In spite of their weak intensity, these absorption bands give supplementary evidence that the P Q equilibrium is not totally shifted towards P tautomer. For 1 (R = (CH3)2CH), no Q absorption band was observed in the CC14 solution spectrume2 Similar conclusions are drawn from the UV-visible spectrum: for 2 (R = (CH&CH) the Q absorption band is observed at 450
-
The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 3719
N-(2,3-Dihydroxybenzylidene)isopropylamine Crystal TABLE III: Frequencies (cm-I) of IR Absorption Bands and Identifications 3548 m 3400 vw 3260 vw
3069 w 2973 m 2934 w 2901 sh 2873 w 1652 w 1635 vs I616 w I600 w 1560 vw 1540 vw 1471 s 1465 s 1455 m
P 3328 vw 3255 w 3212s 3162 w 3123 vw 3040 w 2980 m 2970 m 2930 w
CH,
I
Q
2
1070 w IO30 w 1018 vw
Q Q
1072 w 1068 w IO30 s 1020 s
972 w 950 w
1610 vw
Q"
904 vw 883 w 870 w 840 vw
Ph
PK 950 w 930 w 902 m 880 vw 863 w
I384 m I378 m I365 w 1319 m 1285 sh 1271 vs
1197 1165 sh 1 I54 m 1140 vw 112ovw
Q Qh
P' P'
803 m I408 w
(Y
P
1550 s I520 s 1480 sh I470 s 1460 s 1403 vs 1388 sh
Q
I370 m I360 m 1348 1321 w
Ph
1290
Pb
178 m 760 w 732 vs 724 vs
755 vs 730 m 721 m 702 m
Q PJ PJ
Qh
Q
pd
1260 sh 1240 vs 1230 vs 1220 vs 1 I90 1 I70 sh 1 I65 w 1132s 1125s
Q'
560 w 520 vw
b
580 w 565 w
P"'
460 w
Qf
440 m (.
Q'
518 493 m 440 m
Enamine 11. Pand Q, isopropyl. Ring, 19a. v(C-0). 111. /6(C-H). 8 y(C-H), 5 . 6(N-H). ~ ( 0 - H ) .J ?(C-H). Enamine IV. "I ?(ring), 16b.
Enamine y(N-H).
nm in the crystalline state (KBr pellets; 4/1000by weight); it is also observed, although very weak, in hexane solution at 430 nm (optical density: 0.7 at 5 X mol L-I) whereas it is absent in 1 (R = (CH&CH) spectrum. In our previous publication about the 1 series, we have reported for severalderivatives,including 1 (R = (CH&CH), the presence of Q absorption bands on the IR spectra of crystalline samples. The reported frequencies are very close to the specific Q ones observed for 2 (R = (CH3)2CH).
Theoretical Section TautomerStabilltie. If quantitativedata on P-Q equilibrium are not available, our IR study gives clear answers about the identification of major and minor partners (e): R = (CHj)>CH CC14 solution crystal
AH(P)
AH(Q)
-2.62h.i 2.15 -46.65d-f -43.84n,h
AH(TR) AAH(Q-P) 21.56 4.77 -23.09 2.81
AAH(TR-P) 24.18 23.56
Dihedralangles: $I = C(I)-C(2)-0(2)-H(2);0= C(Z)-C(3)-0(3)H(3). h P i ( ~ = 0 ° ) . ' P z ( ~ = 1 8 0 0AH= ) : 1.08;AAH(P2-P1)=3.70. d pI (6 = 00; e = 00). p2 (6 = 00; e = 1800): AH = -42.59; AAH (p2 - pI) = 4.06. IP, (6 = e = 1800): AH = -41.47; AAH(P~- p i ) = 5.18. 8 QI (e = 0'). 42 (0 = 180'): AH = -39.55; AAH (42- Q I ) = 4.29.
2870 w 1660 sh I642 vs
TABLE IV: Calculated Heats of Formation (kcal mol-') of P and Q Tautomers, Their Transition State (TR), and Their Conformers' by AM1 Method R=
1
P P + fQ
2 P + fQ QQ + cPP + cPQ
Two essential questions arise: (a) Do molecular orbital calculations account for relative stabilization of Q tautomer when a second hydroxyl group is introduced in ortho position? (b)
How can we explain the overstabilizationof Q tautomer in crystals and in water? Ab initio studies of H-bonding systems are very sensitive to basis set and correction for electron correlation. Such calculations performed on molecular complexes of this size are not practicable using such costly methods.sb Therefore, the AM 1 approximation theory has been used for these studies because it is more efficient than MNDO to describe hydrogen bonds. The AMPAC program6 was used for P or Q monomers. Complete geometry optimizations were performed by use of the Davidon-Fletcher-Powell algorithm' (Table I). The transitionstate optimizations were controlled in order to have only one negative eigenvalue in Hessian matrix. All geometrical parameters for each of the monomers were optimized (Table I). For the PP and QQ dimers, all of the geometrical parameters for the second monomer unit were set equal to those of the first. The initial geometry of the dimer was chosen in a way to respect intermolecular hydrogen bonds present in crystal. Then, for 2Pand 24, theoptimizations werecompletely performed to get an optimal dimer A (in Table VI); to better approximate the crystal environment, the aromatic rings of the two monomer units were constrainedto be in parallel planes (dimer B in Table VI). The calculations forecast that, as isolated molecules, the Q tautomers are always less stable than Pones (Table IV), but the insertion of the second hydroxyl group (1 2) significantly decreases the (P - Q) energy difference (4.8 2.8 kcal mol-'). The experimental detection of few percents of 2Q (R = (CH&CH) in CC14 solution at room temperature shows that the calculated 2.8 kcal mol-) is still too large, but the molecules are polar and we have to take into account the solvent effect (the dielectric constant changes from 1.O to 2.0). The simplest way to modelize it requires the use of Kirkwood relation~hip'~ issued from the reaction field theory; as 2 4 is more polar than its 2P tautomer ( p = 5.83 and 3.05 D, respectively, according to AMI calculations for R = CHj), the extrastabilization of 2Q over 2P is about 1.9 kcal mol-), reducing the (P - Q) energy difference to 0.9 kcal mol-', or an 18%Q concentration at room temperature. The real experimental concentration is surely less; our calculations provide a satisfying qualitative forecast. Moreover, such a solvent would also reduce by a similar amount the (P Q) energy difference in the 1 series (AM1: 4.8 kcal mol-'; 3-21G:SC.15 3.9 kcalmol-I), but it wouldnot besufficient toexpect the presence of few percent of 1Q tautomer which actually is absent from the CC14 solution. A recent more sophisticated calculation predicts a larger solvent effect (4.3 kcal mol-l stabilization). I It appears advisable to use the calculated bicentric partition energies to account for the P and Q relative stabilizations associated with the presence of the second hydroxyl group in the 2 series. The interactions between nonbonded atoms displayed on Table V give a rough estimate of hydrogen bond strengths and their differencesmay be used to justify the qualitativeexperimental observations: (1) The 0(2).-H(2) hydrogen bond occurring in Q tautomer is much stronger than the H(2)sv.N ones of P tautomer.
--
3720 The Journal of Physical Chemistry, Vol. 97, No. 15, 1993
Carles et al.
TABLE VI: Calculated Heats of Formation and Interaction Energies (kcal mol-') for Dimers A and B interaction energy" 2P 24
monomer
dimer A
dimer B
A
B
-46.65 -43.84
-96.29 -91.21
-93.30 -90.20
-3.44 -3.53
-0.4 -2.52
Defined as the appropriate energy minus twice the corresponding monomer energy.
and are very similar to the experimental value (1.94 A) in crystal. The heats of formation and hydrogen bonding energies for each dimer are presented in Table VI. The optimal dimer interactions have bonding energiesof similar magnitude, -3.5 kcal mol-' for 2P and 24;the structure B, a relaxed crystal is less stable but stabilization is more important for 24,-2.52 kcal mol-! and only -0.4 kcal mol-' for 2P. This result clearly indicates that packing constraints are able to contribute to increase stability of 2Q dimer. Therefore, the association of molecules in dimers looks to be efficient to shift the P ti Q equilibrium. With two other derivatives of the 2 series (R = C6H5 and O-ClC&), we have observed4the same kind of dimers and of intermolecular hydrogen bonds; the only differencesare the distances between the molecular planes (0.43 and 1.38 A, respectively) and the concentration of Q tautomer which is less than that of 2 (R = (CH3)2CH) crystals. WAVELENGTH (am)
Figure 4. UV-visible absorption spectra of 2 (R = (CH3)2CH): (a) in mol L-I). cyclohexane; (b) in water, ( 1 . 1 X
TABLE V Bicentric Partition Energies (in kcal mol-') between Atoms Involved in the Different Hydrogen Bonds Present in P and Q Tautomers Calculated by AM1 Method (R-CH3) 1P
-12.2 -21.1
1Q 2Pl 2Q I
-12.7 -2 1.4
-12.6 -16.7
(2) The presence of 0(3)H(3) in the 2 molecules does not much change the 0(2)-.H(2) interaction. (3) The 2P 2Q tautomerization induces an important strengthening of the H(3)-.0(2) hydrogen bond (4.1 kcal mol-' stabilization) involved in a five-membered pseudocycle. The last result suggests the use of an hydroxylic solvent to increase 2Q stability. In water, the experimental UV-visible spectrum of 2 (R = (CH3)*CH)(Figure 4) shows the 2P 24 equilibrium to be completely displaced toward the right (A,, = 400 nm; = 3090 L mol-'), where as an apolar solvent shifts the equilibrium toward the left (A,, = 325 nm). In hydroxylic solvents, 2Q might also undergo a conformational change; the new isomer (s-Zwith respect toC(7)-N) might have a zwitterionic character3 and therefore an increased solvation. The intermolecular hydrogen bonds occurring in the crystal4 similarly induce the 2Q stabilization. Calculations using AM 1 method on dimers give the following results: for A dimers of 2P and 2Q the two hydrogen-bond distances are 2.15 and 2.17 A, 2.12 and 2.17 A, respectively, with respect to the initial monomer geometry, only limited modifications occur; the two aromatic rings of the dimers are no more in parallel planes, but dihedral angles of only 2 1O and 1Oo are found for 2P and 2Q,respectively. In comparison to crystallographic data (O(2)-0(2) = 3.29 A; O(3)-0(3)) = 4.40 A the calculated intermolecular distances are longer by 0.3 A for 2P (0(2)-0(2)) and 24 (0(3).-0(3)), equal for 2Q (0(2)-0(2)) and shorter by 0.13 A for 2P (0(3)-.0(3)). For B dimer, the two hydrogen-bond distances are shorter than A dimer ones (1.94 and 2.10 A for 2P, 1.98 and 2.06 A for 2Q)
-
Conclusion With the title compound, we have at our disposal a substrate, the crystalline structure of which is exceptionally shifted toward the quinonic tautomer Q, owing to its dimer association. Experimental and theoretical geometric parameters are in fair agreement. Located in position 3, the 0-H group appears as a tautomeric probe through its IR stretching absorption band; its components show that QQ,PQ, and PP dimers are present. The AM1 computations justify for a part the Q stabilization through the intramolecular hydrogen bonding, but the overstabilization is mainly governed by the intermolecular hydrogen bonding. As an experimental proof, we have shown that the tautomeric equilibrium is completely shifted when the compound isdissolved in water. It is significant that calculations ofoptimized dimer geometries approximate quite well the experimental crystal structure. Such a modeling might be useful as a tool to analyze crystal packing.
References and Notes ( I ) Hadjoudis, E.; Moustakali-Mavridis, 1. Mol. Cryst. Liq. Cryst. 1990, 186, 31-36. (2) Carles, M.; Eloy, D.; Pujol, L.; Bodot, H. J . Mol. Struct. 1987, 156, 43-58. (3) Turbevilie, W.; Dutta, P. K. J . Phys. Chem. 1990, 94, 40604066. (4) Mansilla-Koblavi, F.; Tenon, J. A.; Tour&,S.;Ebby, N.; Lapasset, J.; Carles, M.; Bodot, H. Acta Crystallogr., to be submitted. ( 5 ) (a) Dewar, M. J . S.; Zoebish, E. G.;Healy, E. F.; Stewart, J. J . P. J . Am. Chem. SOC.1985,107,3902. (b) Vinson, L. K.; Dannenberg. J . J. J . Am. Chem. SOC.1989,111,2777-2781. (c) Hofmann. H. J.; Cimiraglia, R.; Bonarccorsi, R.; Unverferth, K.; Tomasi, J. Eur. J . Med. Chem. 1990, 25, 127-130. (6) MOPAC; Stewart, J. J. P.; Eggar, M. Program No.455; Quantum Chemistry Program Exchange. (7) Fletcher, R. Practical Methods of Optimization; Wiley: Chichester. 1981; Vol. I . (8) Mansilia-Kobiavi, F.; Toure, S.;Lapasset, J.; Carles, M.; Bodot, H. Acra Crystallogr. 1989, C45. 451453. (9) (a)Inabe,T.NewJ.Chem.1991, 15, 129-136. (b)Inabe,T.;GautierLuneau, 1.; Hoshino, N.; Okaniwa, K.; Okamoto, H.; Mitani, T.;Nagashima, U.;Maruyama, Y. Bull. Chem. SOC.Jpn. 1991,64, 801-810. (c) Inabe, T.; Hoshino, N.; Mitani, T.; Maruyama, Y. Bull. Chem. SOC.Jpn. 1989, 62. 2245-2251. (d) Hoshino, N.; Inabe, T.; Mitani, T.; Maruyama, Y. Bull. Chem. SOC.Jpn. 1988, 61,42074214. ( 1 0) Tichy, M. Advances in Organic Chemistry; interscience Publishers: New York, 1965; Vol. 5, p 115.
N-(2,3-Dihydroxybenzylidene)isopropylamineCrystal (1 I ) (a) The integrated absorbance of that component actually increases by 50% when the overall absorption band (3530-3080 cm I ) undergoes a 22% increase. These figures must be compared to the limited increase (14%) in the 1700-690-cm I range. Such differences in absorbance variations exclude any quantitative analysis of the tautomeric equilibrium as it has been done for carboxylic acid dimers1Ibwhich does not show differences between sensitivitiesof integrated absorbancecoefficient to temperature. (b) Hayashi, S.; Umemura, J. J . Chem. Phys. 1975, 63, 1732-1740. (12) Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations: McGraw-Hill: New York, 1955. (13) Rao, C. N. R. Chemical Applications of Infrared Spectroscopy; Academic Press: New York, 1963.
The Journal of Physical Chemistry, Vol. 97, No. IS, 1993 3721 (14) (a) Being efficient to forecast solvent effects on conformational equilibrium,"b the Kirkwood relationship
AE5 = -A[(c
- l ) / ( c + l)](p'/a:)
(A€, in kcal mol 1; A = 14.393; c = 2; p in debye; all in A) has been used to calculate AAE, = AE42Q) - AE42P) = -1.92 kcal mol I. To estimate the cavity radius (ao= 3.943 A), we approximate the molecule to a sphere having its volume, this one being the experimental one (61.32 A').' (b) Abraham, R. J.; Bretschneider, E. In Internal Rorarion in Molecules; Orville-Thomas, W.J.. Ed.: Academic Press: London. 1974: DD 481-584. ( I 5 ) Hofmann, H.-J.; Cimiraglia, R.; Tomasi, J. 2.Chem. 1990,30,443444.
