Theoretical study on the intramolecular hydrogen bond in chloro-substituted N,N-dimethylaminomethylphenols. I. Structural effects

Journal of Molecular Structure 700 (2004) 81–90 www.elsevier.com/locate/molstruc

Theoretical study on the intramolecular hydrogen bond in chloro-substituted N,N-dimethylaminomethylphenols. I. Structural effectsq A. Kolla,*, V. Parasukb, W. Parasukc, A. Karpfend, P. Wolschannd a Department of Chemistry, University of Wroclaw, ul. Joliot-Curie 14, Wroclaw PL-50-383, Poland Faculty of Science, Department of Chemistry, University of Chulalongkorn, Bangkok 10330, Thailand c Faculty of Science, Department of Chemistry, University of Kasetsart, Bangkok 10900, Thailand d Institute of Theoretical Chemistry and Structural Biology, University of Vienna, Wa¨hringer Straße 17, Vienna A-1090, Austria b

Received 12 November 2003; revised 2 December 2003; accepted 2 December 2003

Abstract Ab initio and density functional calculations are applied to study the influence of an increasing number of chlorine substituents on the properties of the intramolecular hydrogen bond in substituted Mannich bases. It is shown, that not only the acidity of the proton donor, which depends on the number of chlorine atoms at the aromatic ring, but also steric interactions modify the geometry of the hydrogen bond. Specific interactions of O– Cl· · ·H – O hydrogen-bonding in some derivatives are estimated by calculations on related chlorophenols. q 2003 Published by Elsevier B.V. Keywords: Ab initio calculations; DFT methods; Mannich bases; Intramolecular hydrogen bonding; Substituent effects

1. Introduction Studies on intramolecular hydrogen bonding have become increasingly popular in the past [1 –24]. These systems are attractive because they have a quite high thermodynamic stability and a uniform structure [20 – 25]. The stability of intramolecularly hydrogen-bonded complexes allows to perform experimental studies in a wide range of temperatures and solvents. This feature facilitates the interpretation of proton transfer reaction studies [22,23]. Many investigations on the structure and on the spectroscopy of intramolecular hydrogen bonds are devoted to molecules with p-electronic coupling between the acid and base centres, attached to the same aromatic ring [1 – 19]. In such systems, e.g. Schiff bases, the intramolecular proton transfer reaction proceeds comparatively easily. One can observe double fluorescence [26] as well as thermochromic and photochromic properties [27] related to it. These molecules are potent materials for molecular memory and q This paper was published in error in a previous issue of J. Mol. Struct 690(2004) 165 –174, and now appears as part of this special issue. * Corresponding author. Tel.: þ 48-71-2042-00; fax: þ48-71-2223-48. E-mail address: [email protected] (A. Koll).

0022-2860/$ - see front matter q 2003 Published by Elsevier B.V. doi:10.1016/j.molstruc.2004.07.008

optical switch devices [28] or fluorescent probes of biological systems [29]. In these molecules, intramolecular electron coupling leads to a strengthening of the hydrogen bonds in so-called resonance assisted hydrogen bonds [30]. In the past, we have studied the properties of intramolecular hydrogen bonding in model systems of a more general character—in ortho-hydroxy-N-alkylaminomethylphenols, so called Mannich bases [20 – 24]. Due to the presence of a methylene bridge between proton donor and proton acceptor parts of the molecule, the p-electronic coupling between acid and base centres in these compounds is reduced to a high extent. In the previous works of this series [31,32], the properties of the intramolecular hydrogen bond of the simplest possible Mannich bases, ortho-aminomethylphenol (AP), and N-dimethyl-ortho-aminomethylphenol (DMAP) were studied, applying ab initio and density functional methods for the analysis of the IR spectra and the structure modifications upon the formation of the intramolecular hydrogen bonding. These results have been compared to the properties of the monosubstituted benzene subunits, phenol (PH), benzylamine (BA) and dimethylbenzylamine (DMBA). We have found, however, that the comparison of quantum mechanical calculations of the structure and

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force constants of the hydrogen bonded and related open conformers is a more promising way to study the spectroscopic and structural consequences of intramolecular hydrogen bonding. It was demonstrated that the intramolecular hydrogen bond in Mannich bases reveals changes characteristic for the participation of ortho-quinonoid resonance structures despite the strong reduction of pelectronic coupling within those molecules. The pattern of both the structure and force constants changes was highly consistent. The aim of the present work is to extend the study of the previously established effects of intramolecular hydrogen bond formation to molecules with stronger intramolecular interactions, as a function of increasing acidity of the phenolic part of the molecules. We have selected the four Ndimethylaminomethylphenols substituted with one to four chlorine atoms at the phenol ring: 2-N-dimethylaminomethyl-4-chlorophenol (1), 2-N-dimethylaminomethyl-4,6dichlorophenol (2), 2-N-dimethylaminomethyl-3,4,6-trichloro-phenol (3) and 2-N-dimethylaminomethyl-3,4,5,6tetrachlorophenol (4). The unsubstituted compound (DMAP (0)) was also considered for comparative reasons (Scheme 1). The acidity of the phenolic part of the Mannich bases increases gradually with the number of chlorine atoms [33]. In accordance to that, the values of DpKa ¼ {pKa ðBHþÞ 2 pKa ðAHÞ}; commonly used [34 – 36] in the analysis of the properties of hydrogen bonds, increases too. As the basic part of the molecule (the N-dimethylaminomethyl group) does not change, the influence of the acidic part on the hydrogen bond can be followed directly. We have correlated the calculated structural parameters of the intramolecular hydrogen bond with the experimentally established trends when varying the extent of the chlorine substitution, expressed by DpKa : In the case of chloro-substituted Mannich bases, no experimental information on the gas phase structures is available. A direct comparison with structures derived from the ab initio calculations is, therefore, impossible. Some

Scheme 1.

crystal structures of these compounds are known. The X-ray data were recently reviewed and correlated with the DpKa scale [22]. The detailed comparison of experimental and calculated structures by ab initio and DFT methods will be presented elsewhere [37]. Generally, it was found that the calculated values reproduce the experimental trends quite well and in most cases also the numerical values of the parameters, especially if one considers some increase of the strength of hydrogen bonding in condensed phases due to polarization effects as well as the long range interactions, which are mainly of attractive character.

2. Computations The quantum chemical calculations were performed with the GAUSSIAN 98 program package [38]. Full geometry optimisation was performed at MP2 and B3LYP levels with the 6-31G(d,p) basis set and by B3LYP/D95(d,p).

3. Results and discussion 3.1. Comparison of the structural parameters in the hydrogen bonded and open conformers In previous detailed analyses it was shown [31,32], that convenient measures of the structural changes upon formation of intramolecular hydrogen bonds are the differences between closed and open conformers, where the effect of mutual, electronic interactions between substituents is eliminated to high extent. For these reasons, we have optimised the structures of hydrogen bonded and open forms of the selected Mannich bases with various chlorine substituents. The open forms were obtained by rotation of the O – H bond around the C –O axis by 1808 as well as by rotation of the methylamino group by 1808 around the C2 – C7 bond (Scheme 2). 3.1.1. Bond lengths Previous studies [31,32] have indicated, that the most sensitive structural parameters upon intramolecular hydrogen bond formation are the O –H, C – O, C1 –C2 and C2 – C7 bond lengths. Changes in O – H and C –O bonds are direct consequences of the formation of hydrogen bonds, whereas C1 – C2 and C2 – C7 distance changes can be more related to intramolecular rearrangement and to steric and electronic interactions. If only the acid– base interaction determines the structures, one can expect some smooth correlation of these parameters on DpKa : Steric interactions and specific influence of particular substituents may cause some irregularities for such a correlation. The dependence of the O –H bond length on DpKa for the open and hydrogen bonded conformers, calculated at three different levels of theory, are presented in Fig. 1a. The pKa

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83

Scheme 2.

values used for particular compounds are: 9.92 for phenol, 9.30 for p-chlorophenol, 8.45 for o-chlorophenol, 7.80 for 2,4-dichlorophenol, 7.05 for 2,4,5-trichlorophenol and 6.24 for 2,3,4,5-tetrachlorophenol [33]. The pKa of N-dimethylbenzylamine was taken as equal to 8.9 [39,40]. Evidently, there are separate groups of points for the open and hydrogen bonded conformers. The influence of the number of substituents on the length of the O –H bond in the open forms seems to be rather limited. A small increase of this distance observed for the molecules able to form an alternative hydrogen bond to an ortho-positioned chlorine atom in the open forms of 2, 3 and 4 is probably resulting

from these weak interactions. The average effect of the O – H· · ·N hydrogen bond formation on the O – H bond length is ˚ in DMAP and reaches 0.036 A ˚ in 4. The about 0.026 A tendency to elongation of this bond upon increase of the phenol acidity in hydrogen bonded conformers is evident from the presented correlations. However, to obtain the ‘pure’ effects of the O – H· · ·N hydrogen bonding, the influence of the O –H· · ·Cl hydrogen bonds have to be independently estimated in the open forms of compounds 2 –4. The dependence of C –O bond lengths on DpKa is somewhat smaller but of similar magnitude as for the H –O

˚ ) on DpKa of Mannich bases 0–4. (0 ¼ DMAP, 1 ¼ 4-chloro-DMAP; 2 ¼ 4,6-dichloro-DMAP; Fig. 1. (a) Dependence of calculated O –H bond lengths (in A ˚ ) on DpKa of Mannich bases 0–4. Open 3 ¼ 3,4,6-trichloro-DMAP; 4 ¼ 3,4,5,6-tetrachloro-DMAP. (b) Dependence of calculated C –O bond lengths (in A symbols represent the structural data of the open form of Mannich bases, filled symbols stand for hydrogen-bonded Mannich bases (squares—B3LYP/631G(d,p); circles—B3LYP/D95(d,p); triangles—MP2/6-31G(d,p)).

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˚ per 4pKa units, Fig. 1b), but the bond bond length (0.016 A lengths decrease with increasing DpKa : Our calculations show, that this is not specific for hydrogen bonding, as in the case of the open species similar tendencies as a function of DpKa are observed. Nevertheless, some additional decrease ˚ , resulting from the O – H· · ·N of the order of 0.015 A hydrogen bond formation is observed. One may especially stress a different behaviour the O –H and C – O bond lengths on the substitution. The length of the O – H bond is not sensitive to the number of chlorine substituents unless the group is not hydrogen bonded, while the C –O bond is equally sensitive in both conformers. The position of the points on the graphs (Fig. 1) for the open forms of compounds 2 –4 also depends on the O – H· · ·Cl hydrogen bonding. In order to follow these effects, one should compare to calculations for the open and hydrogenbonded forms of chloro-substituted, related phenols. The C1 – C2 distance, as it was shown previously [32], is affected by two factors. First, there is an increase of the ortho-quinonoid resonance structure in the ground electronic state after formation of intramolecular hydrogen bond. Secondly, an increase of the surroundings rigidity occurs due to formation of the chelate ring. Both effects should lead to an increase of the length of this bond, evidently mirrored a corresponding decrease of the related force constant (cf. [31,32]). In the series of compounds 0 – 4, an increase of ˚ of this bond length is observed (Table 1). 0.007 ^ 0.002 A All applied methods, in average, do not predict any increase of these differences with DpKa in molecules 0 – 2. However, a larger increase is observed for compounds 3 and 4. Some irregular changes in bond lengths observed in the last compounds are resulting probably from steric interaction of chloro-substituents located at position 3. It can influence also the other dependences on DpKa (see further). Generally, the C2 – C7 distances should be shortened upon intramolecular hydrogen bond formation as a consequence of the participation of this bond in orthoquinonoid resonance structure in closed conformers [31]. This was indeed observed for compounds 0 –2. (Table 2). In the contrary, for compounds 3 and 4 an increase of this bond length was obtained. Steric interaction of the chlorine atom substituted at position 3 is probably responsible for

that. Although, the changes of the bond length on chlorine substitution are small, no correlation with DpKa can be seen. O· · ·N and H· · ·N distances are directly related to the strength of the hydrogen bonding, and are commonly used as indicators of hydrogen bond formation. Fig. 2 shows the dependence of O· · ·N and H· · ·N distances on DpKa in the closed forms of Mannich bases. Evidently, there is an increase of the hydrogen bond strength with DpKa : Interestingly, a stronger decrease of both O· · ·N and H· · ·N distances is observed for compounds 3 and 4, where a chlorine substituent is located next to Ndimethylaminomethyl group. Some external squeezing of the chelate ring by this substituent leads to an additional increase of the hydrogen bond strength. This effect can be related to the shortening of the hydrogen bonds in sterically modified ortho-hydroxy Schiff bases [11]. It is not possible to correlate the O· · ·N and H· · ·N distances with DpKa for the open conformers, because they mainly depend on steric interactions, similar to the NCCC torsional angle. 3.2. The effects of the intramolecular hydrogen bonding to ortho-chloro-substituents The results of the structure calculations of all parent (to studied Mannich Bases) phenols and ortho-chlorophenol (applied to increase the data set for correlations) are presented in Fig. 3a and b. Fig. 3a presents the correlation of the O –H distances with the pKa values of the phenols. This parameter is linearly related to DpKa of Mannich bases because of the constant value of pKa (BHþ), benzylamine in our case. The O –H distances in phenol and chlorine substituted phenols show a negligible dependence on their pKa values. There exists a difference between the distances of the open and the hydrogen-bonded forms, but much less than in Mannich bases, indicating a much weaker hydrogen bond between the acidic proton and the chlorine atom in ortho position. A dependence of the C – O distances on the pKa values of phenols can be observed, similar to Mannich bases but significantly smaller. The values for the open phenols (non-hydrogen-bonded forms) are slightly reduced in comparison to the open forms of Mannich bases, those for

Table 1 ˚ C1 –C2 distances in the open and hydrogen-bonded (HB) conformers of chloro-substituted Mannich bases, distances in A Compound

0 1 2 3 4

B3LYP/6-31G(d,p)

MP2/6-31G(d,p)

B3LYP/D95(d,p)

HB

Open

Diff.

HB

Open

Diff.

HB

Open

Diff.

1.413 1.413 1.414 1.417 1.414

1.406 1.406 1.407 1.408 1.405

0.007 0.007 0.007 0.009 0.009

1.410 1.410 1.410 1.412 1.409

1.403 1.403 1.403 1.404 1.401

0.007 0.007 0.007 0.008 0.008

1.417 1.417 1.417 1.420 1.418

1.409 1.409 1.410 1.411 1.409

0.008 0.008 0.007 0.009 0.009

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Table 2 ˚ C2 –C7 distances in the open and hydrogen-bonded (HB) conformers of chloro-substituted Mannich bases, distances in A Compound

0 1 2 3 4

B3LYP/6-31G(d,p)

MP2/6-31G(d,p)

B3LYP/D95(d,p)

HB

Open

Diff.

HB

Open

Diff.

HB

Open

Diff.

1.514 1.515 1.516 1.520 1.520

1.519 1.519 1.520 1.518 1.518

20.005 20.004 20.004 0.002 0.002

1.505 1.505 1.506 1.509 1.509

1.510 1.510 1.511 1.505 1.505

20.005 20.005 20.005 0.004 0.004

1.517 1.518 1.519 1.522 1.522

1.522 1.521 1.522 1.520 1.520

20.005 20.003 20.003 0.002 0.002

˚ ) on DpKa of Mannich bases 0–4. (b). Dependence of calculated N· · ·H distances (in A ˚ ) on DpKa of Fig. 2. (a) Dependence of calculated N· · ·O distances (in A Mannich bases 0 –4. For the description of symbols see Fig. 1.

˚ ) in open and hydrogen-bonded phenols on pKa. (P0 ¼ phenol, P1 ¼ 4-chlorophenol, P1o ¼ 2-chlorophenol, Fig. 3. (a) Dependence of O –H bond lengths (in A ˚ ) in open and hydrogenP2 ¼ 2,4-dichlorophenol, P3 ¼ 2,4,5-trichlorophenol, P4 ¼ 2,3,4,5-tetrachlorophenol. (b) Dependence of C –O bond lengths (in A bonded phenols. Open symbols for free phenols. Black symbols for phenols forming O–H· · ·Cl hydrogen bond. For the description of symbols see Fig. 1.

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˚ ) upon the formation of O–H· · ·N intramolecular hydrogen bonds, corrected for the O– H· · ·Cl interactions in Fig. 4. (a) Increase of the O –H bond length (in A open forms of 3– 5, in function of DpKa : (b) Corrected decrement of C –O bond lengths resulting from the formation of intramolecular hydrogen bond in Mannich bases 0–4 in function of DpKa : For the description of symbols see Fig. 1.

the hydrogen-bonded forms show almost identical behaviour. Taking into account the effects of the O – H· · ·Cl interactions, we were able to calculate the pure effects of the increase of the strength of the O –H· · ·N hydrogen bonding. Fig. 4 presents the corrected, increments in O –H and C – O distances resulting from the formation of O – H· · ·N intramolecular hydrogen bonding as a function of increasing DpKa values. The dependence of O – H bond lengths on ˚ per pKa unit. DpKa has an average slope equal to 0.0024 A Some effects of an additional strengthening of the intramolecular hydrogen bonds by chlorine substitution at position 3 can be seen for compounds 3 and 4. This interaction increases, additionally to above mentioned ˚. correlation, the O –H bond length on 0.005 A There exists a pronounced influence of steric enhancement of intramolecular hydrogen bonding due to interaction of the chlorine atoms at position 3. On the other hand, the effects of the increased acid– base interactions on O –H bond length are stronger than on C –O distance. The slope in Fig. 1b is the result of both—direct influence of substituents on the structure (which is also active in phenols) and the influence of electronic interactions between acid – base centres (related to the DpKa ). 3.3. Phenol ring distortions In the discussion on the structural effects of the intramolecular hydrogen bond formation in AP (orthohydroxyamino-methylphenol) and DMAP we have used some characteristics, which describe changes in the structure of phenol ring. It was found, that the formation

of intramolecular hydrogen bond leads to bond length changes in the phenol rings, to a pattern characteristic for ortho-quinonoid resonance structure, suggesting some kind of electron coupling between substituents despite the preventing function of the methylene bridge [31,32] The effect of bond length modifications can be expressed by means the value of average square of deviations the bond lengths from the mean bond length ðdÞ in the phenyl ring used later as A parameter. X ðdi 2 dÞ2 A¼

i

6

£ 106

The values of the parameter A and the average bond lengths are collected in Table 3 for the aromatic rings of all compounds involved in this study. The average values of bond lengths in the ring, calculated in the procedure of the A evaluation, can be used (Fig. 5) to describe how the substituents influence on the effects of the formation of intramolecular hydrogen bonding. Two different trends are observed. The first concerns the differences between closed (hydrogen bonded) and open forms. The curves for the closed forms of the Mannich bases are located higher than those of the open forms. The difference reflects the effects of charge migration in the direction of the chelate ring formed. Such an effect was demonstrated directly by dipole moment measurements [41, 42]. Generally, the same dependences can be observed for the series of phenols, although the difference between open and closed forms is much smaller there. The second observation concerns the general behaviour of the average C – C ring distance with increasing chlorine

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87

Table 3 ˚ ) and average squares of bonds deviations (A, in A ˚ 2 £ 106) from the average distances Average values of bond distances (d in A Compound

B3LYP/6-31G(d,p) Closed

Benzene N-dimethyl benzylamine Phenol 4-Chloro phenol 2,4-Dichloro phenol 2,4,5-Trichloro phenol 2,3,4,5-Tetra chlorophenol Pentachloro phenol DMAP 4-Cl-DMAP 4,6-DiCl-DMAP 3,4,6-Tri-DMAP 3,4,5,6-Terta-DMAP

d d A d A d A d A d A d A d A d A d A d A d A d A

1.3956 20.6 1.3966 26.6 1.4000 40.9 1.3964 18.3 1.3991 44 1.3981 53.3 1.3987 68.2 1.4013 73.3 1.4049 25.5

MP2/6-31G(d,p) Open

B3LYP/D95(d,p)

Closed

Open

1.3964 1.3972 5.4 1.3965 6.1 1.3954 8.3 1.3959 20.6 1.3969 21.5 1.4005 37.6

1.3976 15.7 1.3964 21.7 1.3965 32.2 1.3994 55 1.4029 6.8

substitution. Relative to benzene, a slight decrease is observed for the first two compounds followed by a stronger increase for the higher substituted compounds. This pattern is observed for the series of Mannich bases as well

Closed

Open

1.3953 13.0 1.3962 19.5 1.3994 34.9

1.3962 1.3971 4.2 1.3959 3.0 1.3950 4.2 1.3956 12.4 1.3965 15.4 1.3997 33.9

1.4011 14.6 1.4018 21.8 1.4047 29.2

1.4027 4.3 1.4013 6.2 1.4015 16.7 1.4023 17.9 1.4053 28.8

1.3986 27.7 1.3977 33 1.3982 42 1.4006 54.2 1.4039 10.8

1.3972 7.3 1.3961 11.2 1.3963 17.5 1.3988 34.5 1.4020 3.2

1.4046 32.7 1.4032 40.8 1.4035 55.7 1.4057 68.7 1.4089 23.5

1.4030 8.7 1.4014 14.4 1.4013 25.2 1.4040 45.5 1.4070 6.3

as for the series of phenols. Comparison with the analogous series of chlorine-substituted benzenes revealed the very same pattern. Mono- and p-dichloro-substitutions lead to ˚ , respectively) reductions of slight (0.001 and 0.002 A

Fig. 5. (a) Dependence of average ring bond lengths in Mannich bases 0– 4 on DpKa : (b) Dependence of average ring C –C bond lengths in chloro-substituted phenols; numbering of phenols is the same as in Fig. 3. For the description of symbols see Fig. 1.

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Fig. 6. (a) Dependence of the parameter A on DpKa for open and closed forms of Mannich bases; for definition of A see text. (b) Dependence of the parameter A on pKa of chlorosubstituted phenols. For the description of symbols see Fig. 1.

the average C – C distance, whereas the more crowded triand tetrachloro-substitutions lead to increases relative to benzene. The effect is strongest in hexachlorobenzene where the calculated average C – C distance is about ˚ larger than in benzene. 0.008 A The variations of the A values for Mannich bases and phenols are given in Fig. 6. From Fig. 6a (cf. also Table 3) it follows, that the distortion of the phenol rings (described by the A parameter) increases with the number of substituted chlorine atoms. Also for the Mannich bases, a substantial increase is seen between the open forms and the closed ones. The last compound in the series, the tetrachloro-substituted Mannich base behaves differently—the A value decreases drastically. This results from the already mentioned increase of the average C –C distance in the higher substituted rings. Phenols (Fig. 6b) have generally lower values for the parameters A; the values of this parameter also increase with the number of substituted chlorine atoms into ring. There are no anomalies in the correlations at tetrachlorophenol. We have additionally determined the structure of pentachlorophenol by B3LYP/6-31G(d,p) method and found that the parameter A is equal to 18 and is more than two times less than for tetrachlorophenol (Table 3), which indicates that a full substitution of the aromatic systems results in a more symmetric ring geometry. Generally, the differences of bond lengths in the rings increase in the series with the number of chlorine substituents, the effect for compounds with intramolecular hydrogen bonding is at least twice as high as than for the related chlorophenols.

4. Summary In the discussion of the structural consequences of the formation of an intramolecular hydrogen bond in chloroderivatives of N-dimethylaminomethylphenol, we intended to analyse the effects of increasing the acidity of the phenolic parts of the molecules, but it appeared that one has to consider carefully a direct effect of the substituents on the structure of the studied compounds in order to explain the structural trends. Because of that, also the modifications of the structural parameters of chlorophenols with DpKa were discussed. The combined analysis of the structure of chlorosubstituted Mannich bases and related phenols leads to the following conclusions concerning the influence of increasing the strength of the intramolecular hydrogen bonds as well as of increasing the number of Cl-substituents on the structural parameters. The O –H distance does not depend on the number of the chlorine substituents in the phenols and Mannich bases, if this group is not engaged in hydrogen bonding, e.g. in the open conformers. When the O – H· · ·N hydrogen bonds are formed in the case of the Mannich bases, the O –H distance becomes strongly dependent on the number of introduced substituents, in a way parallel to the increased acidity of the phenol. The observed effect of O –H bond extension in Mannich bases is of the ˚ when DpKa increases by four units (cf. order of 0.015 A Fig. 4a). A dependence on the number of substituents was not observed for the O –H· · ·Cl hydrogen bonds. There, the effect is one order of magnitude smaller in

A. Koll et al. / Journal of Molecular Structure 700 (2004) 81–90

phenols forming intramolecular O – H· · ·Cl hydrogen bonds (cf. Fig. 3a). Contrary to this, very distinct effects resulting from both, the formation of the hydrogen bond and the number of chlorine substituents are observed for the C –O distance in the open as well as in the closed forms of Mannich bases. The average distance between correlation lines for the open ˚ , which can be taken and closed forms amounts to 0.0015 A as average effect of the formation of intramolecular O – H· · ·N hydrogen bond. The O· · ·N and H· · ·N distances show a typical correlation on DpKa ; demonstrating a direct dependence of the strength of hydrogen bonds on the number of chlorine atoms (through changes of pKa values). Nevertheless, a discontinuity of the correlation lines is observed (Fig. 2), suggesting an additional increase of the strength of intramolecular hydrogen bonds. Similar effects are seen when considering the dependence of O – H and C – O distances on DpKa (Fig. 4). Some irregularities are also observed in dependence of C1 – C2 and C2 –C7 bond lengths on DpKa ; which can be attributed to steric effect of chlorine atom substitution at position 3 of the phenol ring of compounds 3 and 4. The calculations demonstrate, that this steric interaction of the substituent located next to the methylamino group squeezes externally the chelate ring, leading to a pronounced increase of the strength of the intramolecular hydrogen bond. Parameters d; the average bond lengths in the rings, and A; bond length distributions, were applied to describe the perturbations of the ring geometry. The average bond distances show an unusual parabolic dependence on DpKa ; both for the open and hydrogen-bonded conformers, as well as for the phenols. Changes of d; after correction for the direct substituent effect, suggest the charge flow in the direction of the new-formed chelate ring. This effect does not depend pronouncedly on DpKa : The bond length distribution becomes less uniform upon increasing the number of chlorine atoms, but is twice so strong in the systems with O – H· · ·N hydrogen bonding than for the open conformers.

Acknowledgements ¨ AD for financial support The authors wish to thank the O within the Polish – Austrian exchange program (18/2002), the ZID of the University Vienna and the University of Wroclaw for computational facilities.

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