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Steric enhancement of the strength of intramolecular hydrogen bond in 3-Cl substituted 2-(N-dimethylaminomethyl) phenols
Chemical Physics Letters 370 (2003) 74–82 www.elsevier.com/locate/cplett
Steric enhancement of the strength of intramolecular hydrogen bond in 3-Cl substituted 2-(N-dimethylaminomethyl) phenols P. Lipkowski a
a,b,1
, A. Koll
b,*
, A. Karpfen c, P. Wolschann
c
Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland b Institute of Chemistry, University of Wroclaw, ul. Joliot-Curie 14, PL-50383, Poland c Institute of Theoretical Chemistry and Molecular Biology, Universit€at Wien, W€ahringer Straße 17, Vienna, A-1090 Austria Received 9 September 2002; in final form 6 January 2003
Abstract The structure modification and energetic consequences of the formation of intramolecular hydrogen bonds in the series of chloro substituted 2-(N-dimethylaminomethyl) phenol are discussed on the basis of result of ab initio and DFT calculations. A specific behaviour of derivatives containing the 3-Cl substituent was detected, suggesting formation of the stronger hydrogen bond than in analogous derivatives without such a substituent. It was found that steric repulsion of 3-Cl atom moves the methylamino group towards the OH group, leading to shortening of the OH N hydrogen bridge. The geometric and electronic consequences of such shortening of the hydrogen bond are similar to the effects of the increase of acid–base interactions. The estimation of the corrections on steric interaction in the procedure of the energy of intramolecular hydrogen bond calculation appears to be much more complicated than in the series of the compounds not containing the 3-Cl substituent. Ó 2003 Published by Elsevier Science B.V.
1. Introduction Derivatives of N-alkyloaminomethyl phenols are able to form six member rings of intramolecular hydrogen bonds, which due to their stability are model system in the study of the properties of hydrogen bonds and the proton transfer equilib-
*
Corresponding author. Fax: +48-71-328-23-48. E-mail addresses: [email protected] (P. Lipkowski), [email protected] (A. Koll). 1 Also corresponding author. Fax: +48-71-322-348.
rium in dependence on solvent and temperature modifications [1]. This reaction, so important in many biological systems, can be studied in intramolecular hydrogen bonds much more unambiguously [1,2] than in intermolecular system, where the change of the surroundings or concentration may lead to easy modification of the structure and constitution of the complexes [3]. On the other hand such hydrogen bonds can reveal specific properties [4], which should be thoroughly investigated to establish which features of such hydrogen bonding are of general importance, also valid for intermolecular complexes.
0009-2614/03/$ - see front matter Ó 2003 Published by Elsevier Science B.V. doi:10.1016/S0009-2614(03)00068-X
P. Lipkowski et al. / Chemical Physics Letters 370 (2003) 74–82
In the study of the consequences of the formation of intramolecular hydrogen bond the theoretical approach becomes popular, in which the calculated features of hydrogen bonded structures are compared with the properties of components of the system or with ÔopenÕ, nonhydrogen bonded conformers [5–15]. Resembling approach in the study of the intramolecular interactions and spectroscopic force field in derivatives of phenol was applied [15,16]. In the papers [11,12] the structural effects of the formation of intramolecular hydrogen bond in 2-aminomethylphenol (AP) and 2-(N-dimethylaminomethyl) phenol (DMAP) were studied by comparison of the structure of hydrogen bonded and open conformers. Increase of the ring C–C bonds differentiation upon the hydrogen bond formation was found, with the pattern characteristic for ortho quinoid structure. It was also found that force constants in internal coordinates show the same features. In [14] the structure effect of the intramolecular hydrogen bond in chloro substituted Mannich bases in the function of increasing number of substituents was studied. It was demonstrated that these effects depend not only on acid–base properties of the active groups, but also on specific resonance and inductive interactions between substituents. One of the specific characteristics of intramolecular hydrogen bond is its energy (DEHB ) [7– 9,13,17]. Estimation of DEHB appears to be not a trivial task, as it requires definition of the reference state and estimation of the effects of additional interaction between substituents of inductive, resonance and steric character. The first problem reveals because separate components of the complex are not accessible. One of the three possible conformers with broken hydrogen bridge is in the use of such reference state, mostly often obtained by rotation of OH group on 180°. In [13] it was shown that more proper is the use of the state with the lowest energy, when both OH group and a basic component (dimethylamino group in our case) are rotated out of the bonded conformation. The second problem – the corrections for steric interaction changes upon the chelate ring formation was solved for the group of substi-
75
tuted Mannich bases [13]. The partition of calculated energy of four different conformers was performed and steric effects connected with repulsion of OH and CH2 NðCH3 Þ2 group were estimated on 1–1.5 kcal/mol, respectively. It appeared that adding about 2 kcal/mol to the energy difference between I and IV conformers gives the corrected energy of intramolecular hydrogen bond formation, which can be compared with the energy of intermolecular interaction, and correlates precisely with such characteristics of hydrogen bond as for example N HðOÞ distance or charge density in the critical point of hydrogen bond obtained with the help of AIM method [13]. In this work we attempt to extend previous considerations to a new group of chloro derivatives of Mannich bases which contain the Cl substituent in 3 – position, neighbour to dimethylaminomethyl group. This substituent introduces radical changes in the scheme of interaction. The main effect is strengthening of the intramolecular hydrogen bond. The aim of this work is detailed characterization of this effect, which is similar to found in Schiff bases [18–20], where strong enhancement of the hydrogen bond was found as well as modification of the potential for the proton transfer within the hydrogen bridge. Such steric effects appeared to be a factor allowing access to proton transfer forms so important for photophysic properties of these systems like ground and excited state proton transfer, double fluorescence [21], thermochromism [22], or some biological activity [23]. The importance of steric effects of chloro substituents enhancing the strength of intramolecular hydrogen bond was not discussed in literature so far. We shall perform calculations by MP2/6-31G** method and DFT method at B3LYP/6-31G** and B3LYP/d95** levels for all four conformers shown in Scheme 1 for the compounds presented in Scheme 2. The scheme shows also the labelling of the compounds not containing 3-Cl substituent, used further for comparison. Direct results of energy calculations for these substances were already presented [13]. Besides the analysis of the structural effects of Cl substitution at position 3, on the
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P. Lipkowski et al. / Chemical Physics Letters 370 (2003) 74–82
Scheme 1. Thermodynamic cycle describing the partition of the energy of intermolecular hydrogen bond.
Scheme 2. Definition of labelling system in different chloro substituted of N-(dimethylaminomethyl) phenol.
characteristics of intramolecular hydrogen bond in m-chloro substituted Mannich bases, we aim also to compare the effectiveness of the scheme of hydrogen bond energy estimation, proposed in the previous [13] publication.
2. Computations The quantum chemical calculations were performed with the GA U S S I A N 98 program package [24]. Full geometry optimization was performed at MP2 and B3LYP levels with 6-31G** basis set as well as at B3LYP/d95** level. In [25] it was demonstrated that such basis set gives sufficiently good reproduction of the structure of the compounds of similar size. The AIM [27] analysis has been performed with the AIM2000 code [28], with all default options.
3. Results and discussion 3.1. The structural effects resulting from the formation of intramolecular hydrogen bond Such studies were performed already for a group of four chloroderivatives of N-dimethylaminomethyl phenol (DMAP) and DMAP itself [14]. The aim of these studies was to establish how the structural consequences of the formation of intramolecular hydrogen bond depend on the acidity of the phenol part of intramolecular complex. It was found that increase of DpKa [¼ pKa ðBHþ Þ pKa ðAHÞ, for the AH B hydrogen bond] leads to extension of OH bond length DdðOHÞ and shortening of the CO bond length DdðCOÞ, like in intermolecular interaction. The DdðOHÞ and DdðCOÞ values are the differences between dðOHÞ or dðCOÞ in hydrogen
150.3 150.3 151.0 151.2 151.4 151.1 151.9 151.3 151.4 151.9 152.4 A, B3LYP/6-31G(d,p); B, MP2/6-31G(d,p); C, B3LYP/d95(d,p).
Av. C B
148.9 149.0 149.7 149.8 150.2 149.8 150.6 149.8 150.2 150.7 151.1 149.0 149.2 149.9 150.2 150.3 150.0 150.9 150.3 150.3 151.0 151.5
A Av.
1.3610 1.3587 1.3527 1.3560 1.3503 1.3510 1.3480 1.3541 1.3493 1.3466 1.3451 1.3617 1.3599 1.3536 1.3567 1.3512 1.3522 1.3487 1.3551 1.3506 1.3478 1.3461
C B
1.3638 1.3615 1.3560 1.3592 1.3535 1.3542 1.3516 1.3574 1.3524 1.3501 1.3486 1.3574 1.3548 1.3486 1.3521 1.3463 1.3467 1.3437 1.3498 1.3450 1.3420 1.3405
A Av.
2.7100 2.7008 2.6865 2.6599 2.6777 2.6792 2.6401 2.6461 2.6735 2.6258 2.6190 2.6960 2.6890 2.6760 2.6495 2.6687 2.6696 2.6322 2.6360 2.6646 2.6218 2.6127
C B
2.7099 2.7004 2.6853 2.6633 2.6746 2.6778 2.6403 2.6506 2.6710 2.6229 2.6198 2.7241 2.7130 2.6983 2.6669 2.6897 2.6902 2.6477 2.6517 2.6850 2.6326 2.6245
A Av.
1.8084 1.7971 1.7757 1.7457 1.7624 1.7664 1.7170 1.7294 1.7571 1.7013 1.6885 1.7845 1.7758 1.7560 1.7255 1.7448 1.7476 1.7002 1.7094 1.7395 1.6877 1.6734
C B
1.8142 1.8028 1.7801 1.7564 1.7642 1.7709 1.7234 1.7416 1.7597 1.7072 1.6959
A C
0.9970 0.9984 1.0009 1.0034 1.0030 1.0024 1.0076 1.0053 1.0040 1.0095 1.012 0 1a 1b 1c 2a 2b 2c 2d 3a 3b 4
Av.
0.9892 0.9908 0.9934 0.9945 0.9957 0.9949 0.9995 0.9964 0.9967 0.9975 1.0036 0.9908 0.9925 0.9949 0.9972 0.9969 0.9966 1.0015 0.9992 0.9981 1.0039 1.0059
0.9923 0.9939 0.9964 0.9984 0.9985 0.9980 1.0029 1.0003 0.9996 1.0036 1.0072
B A
1.8264 1.8127 1.7911 1.7552 1.7781 1.7807 1.7274 1.7373 1.7722 1.7091 1.6962
Angle ðOH NÞ rCO rNO rNH rOH
Table 1 The values of the calculated O–H, C–O bond lengths and OH N and O N distances (kcal/mol), for hydrogen-bonded conformer
bonded and open conformers, respectively. The corrections in dðOHÞ and dðCOÞ in the open forms of Mannich bases, resulting from the formation of OH Cl hydrogen bond to o-Cl, atom were made. The calculations of the structure in OH Cl, hydrogen bonded and open forms of related phenols were performed in such a case, cf. [14]. Nevertheless, even after these corrections no linear correlation of DdðOHÞ and DdðCOÞ on DpKa was found [14]. Especially large deviations from these correlations were found, when passing from compounds with 3-Cl substituent to a group of derivatives without this substituent. In this Letter the same calculations were performed for the next six chloro derivatives 2-(N-dimethylaminomethyl) phenol. Table 1 contains the calculated values of distances, which are the most sensitive to modification of hydrogen bond strength, for all further discussed Mannich bases. CompoundsÕ labelling is the same as in Scheme 2. Figs. 1a,b show the dependence of corrected (as described previously, cf. also [14]) increase of dðOHÞ and decrease of dðCOÞ upon the intramolecular hydrogen bond formation as a function of DpKa for all the 11 compounds defined in Scheme 2, averaged over three applied here theoretical methods. One can see a regular increase of these parameters on DpKa . pKa values were taken from [26]. The derivatives with 3-Cl substituents form definitely a separate class of hydrogen bonds, which can be characterized as stronger ones because of larger DdðOHÞ and DdðCOÞ at a particular value of DpKa . This we assigned to steric repulsion of 3-Cl substituent, pushing N-methylaminemethyl group in the direction of OH, and making the hydrogen bond shorter and stronger. Such effects are widely described in orthohydroxy Shiff bases, where external squeezing leads to shortening of the hydrogen bond [18–20]. In a previous publication [13] it was demonstrated that some characteristics of hydrogen bonds can be estimated from the calculation within the AIM model. It was shown that the structural consequences of the hydrogen bond formation well correlate with such parameters like the charge density in bond critical point. It was found that for all 11 compounds discussed in this Letter there exists almost linear, uniform depen-
77
149.4 149.5 150.2 150.4 150.6 150.3 151.1 150.5 150.6 151.2 152.0
P. Lipkowski et al. / Chemical Physics Letters 370 (2003) 74–82
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P. Lipkowski et al. / Chemical Physics Letters 370 (2003) 74–82
Fig. 2. Dependence of the charge density qðrÞ at critical point of H N bond on DpKa . j, Points from the compounds with 3-Cl substituent; r, without this substituent.
intramolecular hydrogen bond is stronger in compounds containing the 3-Cl substituents. Both lines have a positive slope in dependency on DpKa . These results well coincide with previously discussed structural effects. Fig. 1. (a) Dependence of the OH bond increment resulting from formation of intramolecular hydrogen bond on DpKa . j, Points from the compounds with 3-Cl substituent; r, without this substituent. (b) Dependence of the CO bond increment resulting from formation of intramolecular hydrogen bond on DpKa . j, Points from the compounds with 3-Cl substituent; r, without this substituent.
dence of charge density in critical point qðrÞ of OH N bond on the dðN HÞ distance. It demonstrates that electron properties of hydrogen bond depend basically on the N H distance despite the fact that there are two different sources for this distance shortening. One is the acid–base interaction, the second is steric shortening of N H. The linear correlation of qðrÞ ¼ a þ bdðN HÞ can be described by the parameters: a ¼ 0:1164, b ¼ 0:2544, R2 ¼ 0:9973. One can use the ÔlocalÕ property of hydrogen bond (charge density in critical point) to characterize its strength (cf. [13,17]). This can be expected as universal measure of the strength of intra- and intermolecular complexes. Fig. 2 presents a dependency of qðrÞ on DpKa . It clearly shows that for series of compounds with m-Cl substituent, the charge in the critical point is substantially higher than that for compounds without 3-Cl substituent. This result can be taken as a direct evidence that
3.2. Energetic consequences of the intramolecular hydrogen bond formation The results of the energy calculations are presented in Table 2. The difference between I and IV forms (DE) is a definition of ÔthermodynamicÕ energy of hydrogen bond, which after some consideration, like zero point energy corrections, can be 0 compared with experimental DHHB values if such are accessible (when open forms are observed in experiment). The obtained values of DE do not correlate with DpKa , which is mainly resulting from the decrease of the energy of reference (IV) state for molecules containing also Cl substituent at 6 position, where intramolecular hydrogen bond of OH Cl type is formed. To correct the energy value on this effect we have performed the calculations for related phenols in the bonded (OH Cl) and open forms 0 (DEðOH ClÞ ). The results are presented in Table 4. Having in disposal the result of calculations of the energy of four different conformers (Table 2) of the compounds able to form intramolecular hydrogen bond one can attempt to estimate the energy of intramolecular hydrogen bond [13]. As it
P. Lipkowski et al. / Chemical Physics Letters 370 (2003) 74–82
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Table 2 The calculated energy differences between particular conformers defined in Scheme 1 (kcal/mol) EI EII
1c 2c 2d 3b 4
EI EIII
EII EIV
EIII EIV
A
B
C
A
B
C
A
B
C
A
B
C
)9.84 )7.29 )10.11 )7.63 )7.65
)9.78 )7.55 )10.01 )7.83 )7.91
)10.59 )7.62 )10.85 )7.96 )7.91
)9.39 )10.05 )9.53 )10.17 )10.37
)9.47 )10.12 )9.55 )10.20 )10.45
)9.89 )10.46 )9.99 )10.54 )10.69
0.37 0.39 0.48 0.49 0.49
0.59 0.62 0.70 0.73 0.73
0.27 0.29 0.39 0.38 0.43
)0.08 3.15 )0.10 3.04 3.20
0.28 3.18 0.25 3.09 3.27
)0.43 3.13 )0.45 2.95 3.20
A, B3LYP/6-31G(d,p); B, MP2/6-31G(d,p); C, B3LYP/d95(d,p).
was mentioned previously no direct measures of DEHB are available experimentally (no open forms are accessible) nor theoretically (one cannot define the reference state with infinitively separated interacting fragments) which is a norm in intermolecular interactions. The calculations proceed according to Scheme 1. Two general possibilities can be accounted. In the case of 3-Cl substituted compounds the scheme does not give a direct measure of the desired property. Additional to all parameters used in the cases of compounds without m-Cl substitution, which are O–H Cl interaction energy DEðO–H ClÞ, steric interaction of –OH ðS2 Þ and –CH2 –NðCH3 Þ2 groups (S1 ) there is also steric interaction of the m-Cl substituent on benzylamine moiety in open conformers III and IV (S3 ). Some changes of S1 and S2 parameters can be expected because of probably different steric interactions also in I and II conformers. Because it is not possible to estimate all these parameters independently, one can use some simplification assuming S1 parameter as for molecules discussed in previous Letter [13]. S1 parameter was changing regularly with the number of chlorine substituents and the average values for the molecules with the same number of chlorine atoms were applied. The value of S1 for 4 was taken from extrapolation in function of the number of substituents, for each calculation method separately. The differences between the energies of II and IV conformers are equal to S1 S3 . In comparison to differences for related molecules without 3-Cl substituent this value is about 1 kcal/mol smaller, what results from compensation by S3 . It allows also direct estimation of S3 which is given in Table 3. This effect of the repulsion of dimethylbenzylamine
group with 3-Cl substituent can be estimated independently, may be more directly (S30 values in Table 3), by calculations of the energy of two conformers of properly Cl-substituted N-benzylamines. Of course then, the difference DEðII IVÞ gives straight the alternative values of S10 , which has the same meaning as S1 , but obtained by using S30 values; S10 ¼ DEðII IVÞ þ S30 . The results are given in Table 3. As one can mention, the results of both ways of S1 and S3 calculations are quite similar, what allows one to believe that those values are estimated with precision not worse than 0:3 kcal/mol, which for such calculations seem pretty good. In further consideration we use the S10 and S30 values as obtained in a more direct way. S2 values were estimated in the previous paper to be 0.9–1.0 kcal/mol, for compounds not containing the 3-Cl substituent. For some of the compounds it was directly known from the difference in energy between III and IV states (cf. Scheme 1) and for others corrected on DEðOH ClÞ0 interactions. In the case of compounds with substituent at 3 position, such calculated values of S20 are considerably lower, from )0.1 to 0.14 kcal/mol (Table 3). This result seems to be very surprising, because one can expect even Table 3 Steric corrections to the energy of intramolecular hydrogen bond (kcal/mol); see text
1c 2c 2d 3b 4
S1
S10
S2
S20
S3
S30
1.26 1.38 1.38 1.48 1.65
1.43 1.63 1.68 1.86 1.94
)0.08 0.88 )0.10 0.88 0.88
)0.08 0.14 )0.10 0.13 0.12
0.85 0.95 0.86 0.95 1.10
1.02 1.25 1.10 1.33 1.40
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P. Lipkowski et al. / Chemical Physics Letters 370 (2003) 74–82
some increase of steric repulsion resulting from introducing the (O)H atom into nearer contact to benzylamine group. This problem will be discussed later on. Having at oneÕs disposal all necessary corrections one can calculate the DEHB values. In the first step one calculates DEðOH NÞ ¼ DE þ DEðOH ClÞ þ S3 ; which is the energy of intramolecular hydrogen bond calculated as a difference in energy between I and IV states, when the last one was corrected on intramolecular hydrogen bond of OH Cl type in the open form of OH Cl type if molecules contain the 6-Cl substituent. In the next step one obtains DEHB from the dependence:
Fig. 3. Dependency of the energy of hydrogen bond, defined as DEðOH NÞ – (see text) on DpKa . j, Points from the compounds with 3-Cl substituent; r, without this substituent.
DEHB ¼ DEðOH NÞ S1 S2 : It can also be calculated using the equation (Scheme 1): DE ¼ DEHB þ S1 þ S2 S3 DEðOH ClÞ: The results are collected in Table 4. They are compared with the results for compounds without 3-Cl substituent in Figs. 3 and 4. From Fig. 3 it is seen that the calculated energy of intramolecular hydrogen bond increases with DpKa . The line for the compounds with 3-Cl substituent is located higher than that for other compounds, which is in full agreement with previously found conclusions based on structural and electronic (qðrÞ) evidences. Surprisingly, the strength of hydrogen bond (Fig. 4) for molecules without 3-Cl substituent appears higher (for a given DpKa value) than that for compounds with 3-Cl. These results contradict to all previously obtained conclusions, showing the strengthening of intramolecular hydrogen bond by steric interactions of 3-Cl substituent. The difference seems to result from problems with the esti-
Fig. 4. Dependency of the calculated energy of intramolecular hydrogen bond in chloro substituted Mannich bases on DpKa . j, Points from the compounds with 3-Cl substituent; r, without this substituent.
mation of S2 correction. If one accepts the values from [13] one obtains the relations between DEHB for both groups of compounds in agreement with all previous reasoning. Estimation of DEðOH NÞ does not require any corrections, but DEðOH ClÞ and S3 . The S1
Table 4 The estimated values of the energy of intramolecular hydrogen bond DEHB and its components (kcal/mol); see text
1c 2c 2d 3b 4
DE
DEðOH ClÞ
DEðOH ClÞ0
DEðOH NÞ
DEðOH NÞ0
DEHB
)9.66 )7.06 )9.78 )7.28 )7.28
– )2.27 – )2.15 )2.35
– )3.01 – )2.90 )3.10
)8.81 )8.38 )8.92 )8.48 )8.53
)8.64 )8.97 )8.68 )8.85 )8.98
)9.99 )10.64 )10.22 )10.83 )11.04
P. Lipkowski et al. / Chemical Physics Letters 370 (2003) 74–82
values are very much resembling these obtained in [13]. S30 values obtained from calculations for benzylamines are of similar order. Only S20 for compounds with 3-Cl substituent are smaller by nearly 1.0 kcal/mol than in [13]. For compounds 1c and 2d they result directly from the difference in energy between III and IV states and possible errors in the DðOH ClÞ estimation do not influence the results. The main difference seems to result from using the different reference states III and IV in both series of chloro substituted DMAP. In compounds with 3-H substituent, the N–C7 –C2 –C3 torsional angle is 25° [14], while in 3Cl substituted compounds it is 75°. The OH group interacts in a different way with the ÔopenÕ N-dimethylmethyl group. A careful comparison of the distances between ÔnonbondingÕ interacting atoms in the vicinity of the chelate ring does not show any distinct differences between two groups of discussed compounds. The difference in energy of III conformers with the geometry obtained for the molecules with 3-H- and 3-Cl substituents is less than 0.1 kcal/mol, however. The only difference seems to be the CH Cl contact, which appears to be shorter in conformer III than in IV on (B3LYP/6-31G**). If it has an attractive 0.06 A character, the cancellation of repulsions (similar like in [13]) and increased H Cl attractions can make the energy difference between III and IV conformers so small, as it follows from calculations. Obtained in this work is the result on estimation where the corrections on steric interactions appeared to be much more complicated than in previously [13] analysed set of compounds. It demonstrates that it is very hard to obtain the systems, where simple portioning of the energy and assuming the additivity of corrections are sufficient to obtain a reliable estimation of the energy of the intramolecular hydrogen bond.
4. Conclusions The results of the energy calculations for four conformations of chloro derivatives of 2-(N-dimethylaminomethyl) phenols at ab initio and DFT levels show that the Cl substituent at position 3 in a
81
specific way influences the structure and energy of the intramolecular hydrogen bond. It was demonstrated that steric interaction of this substituent shortens the hydrogen bridge, which makes the hydrogen bond stronger. A more general conclusion goes from performed calculations; independently of the sources of the hydrogen bond shortening – the acid–base attraction or external steric squeezing the charge distribution in H N bond is the same for a system with a given H N distance. Concerning the estimation the energy of intramolecular hydrogen bond, the calculations for the system without 3-Cl substituent seem to give reliable values while the change of the conformation of reference states in 3-Cl-derivatives makes such discussion too complicate. Very delicate changes in geometry introduce so large differences in the energy that it is very hard to obtain an univocal solution. Acknowledgements € AD for financial supThe authors thank the O port within Polish – Austrian exchange program 22/2000, the Cracow Supercomputer Centre and Wroclaw Centre for Networking and Supercomputing. References [1] A. Koll, P. Wolschann, Monatsh. Chem. 130 (1999) 983. [2] T.S. Humble, Ch.J. Halkides, J.D. Keltner, M. Messina, Chem. Phys. Lett. 298 (1998) 90. [3] M. Szafran, J. Chem. Soc. Faraday Trans. 2 (88) (1992) 1261; J. Chem. Soc. Faraday Trans. 90 (1994) 2489. [4] A. Filarowski, A. Koll, Vib. Spectrosc. 17 (1998) 123. [5] K.B. Borisenko, I. Hargittai, J. Mol. Struct. (Theochem) 388 (1996) 107. [6] K.B. Borisenko, C.W. Bock, I. Hargittai, J. Phys. Chem. 100 (1996) 7426. [7] M. Cuma, S. Scheiner, T. Kar, J. Am. Chem. Soc. 120 (1998) 10499. [8] M. Cuma, S. Scheiner, T. Kar, J. Mol. Struct. (Theochem) 467 (1999) 37. [9] M. Fores, S. Scheiner, Chem. Phys. 246 (1999) 65. [10] A. Koll, M. Rospenk, E. Jagodzinska, T. Dziembowska, J. Mol. Struct. 552 (2000) 193. [11] S.M. Melikova, A. Koll, A. Karpfen, P. Wolschann, J. Mol. Struct. 523 (2000) 223.
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[12] A. Koll, S.M. Melikova, A. Karpfen, P. Wolschann, J. Mol. Struct. 559 (2001) 127. [13] P. Lipkowski, A. Koll, A. Karpfen, P. Wolschann, Chem. Phys. Lett. 360 (2002) 256. [14] A. Koll, V. Parasuk, W. Parasuk, P. Wolschann, A. Karpfen, J. Mol. Struct. (Theochem), submitted. [15] O. Tishchenko, E.S. Kryachko, M.T. Nguyen, Spectrochim. Acta A 58 (9) (2002) 1951. [16] W. Zierkiewicz, D. Michalska, Th. Zeeqers-Huyskens, J. Phys. Chem. A 104 (2000) 11685. [17] S. Grabowski, J. Mol. Struct. 562 (2001) 137. [18] A. Filarowski, A. Koll, T. Głowiak, Monatsh. Chem. 130 (1999) 1097. [19] A. Filarowski, T. Głowiak, A. Koll, J. Mol. Struct. 484 (1999) 75. [20] A. Filarowski, A. Koll, T. Głowiak, J. Chem. Soc. Perkin Trans. 2 (2002) 835.
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Steric enhancement of the strength of intramolecular hydrogen bond in 3-Cl substituted 2-(N-dimethylaminomethyl) phenols P. Lipkowski a
a,b,1
, A. Koll
b,*
, A. Karpfen c, P. Wolschann
c
Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland b Institute of Chemistry, University of Wroclaw, ul. Joliot-Curie 14, PL-50383, Poland c Institute of Theoretical Chemistry and Molecular Biology, Universit€at Wien, W€ahringer Straße 17, Vienna, A-1090 Austria Received 9 September 2002; in final form 6 January 2003
Abstract The structure modification and energetic consequences of the formation of intramolecular hydrogen bonds in the series of chloro substituted 2-(N-dimethylaminomethyl) phenol are discussed on the basis of result of ab initio and DFT calculations. A specific behaviour of derivatives containing the 3-Cl substituent was detected, suggesting formation of the stronger hydrogen bond than in analogous derivatives without such a substituent. It was found that steric repulsion of 3-Cl atom moves the methylamino group towards the OH group, leading to shortening of the OH N hydrogen bridge. The geometric and electronic consequences of such shortening of the hydrogen bond are similar to the effects of the increase of acid–base interactions. The estimation of the corrections on steric interaction in the procedure of the energy of intramolecular hydrogen bond calculation appears to be much more complicated than in the series of the compounds not containing the 3-Cl substituent. Ó 2003 Published by Elsevier Science B.V.
1. Introduction Derivatives of N-alkyloaminomethyl phenols are able to form six member rings of intramolecular hydrogen bonds, which due to their stability are model system in the study of the properties of hydrogen bonds and the proton transfer equilib-
*
Corresponding author. Fax: +48-71-328-23-48. E-mail addresses: [email protected] (P. Lipkowski), [email protected] (A. Koll). 1 Also corresponding author. Fax: +48-71-322-348.
rium in dependence on solvent and temperature modifications [1]. This reaction, so important in many biological systems, can be studied in intramolecular hydrogen bonds much more unambiguously [1,2] than in intermolecular system, where the change of the surroundings or concentration may lead to easy modification of the structure and constitution of the complexes [3]. On the other hand such hydrogen bonds can reveal specific properties [4], which should be thoroughly investigated to establish which features of such hydrogen bonding are of general importance, also valid for intermolecular complexes.
0009-2614/03/$ - see front matter Ó 2003 Published by Elsevier Science B.V. doi:10.1016/S0009-2614(03)00068-X
P. Lipkowski et al. / Chemical Physics Letters 370 (2003) 74–82
In the study of the consequences of the formation of intramolecular hydrogen bond the theoretical approach becomes popular, in which the calculated features of hydrogen bonded structures are compared with the properties of components of the system or with ÔopenÕ, nonhydrogen bonded conformers [5–15]. Resembling approach in the study of the intramolecular interactions and spectroscopic force field in derivatives of phenol was applied [15,16]. In the papers [11,12] the structural effects of the formation of intramolecular hydrogen bond in 2-aminomethylphenol (AP) and 2-(N-dimethylaminomethyl) phenol (DMAP) were studied by comparison of the structure of hydrogen bonded and open conformers. Increase of the ring C–C bonds differentiation upon the hydrogen bond formation was found, with the pattern characteristic for ortho quinoid structure. It was also found that force constants in internal coordinates show the same features. In [14] the structure effect of the intramolecular hydrogen bond in chloro substituted Mannich bases in the function of increasing number of substituents was studied. It was demonstrated that these effects depend not only on acid–base properties of the active groups, but also on specific resonance and inductive interactions between substituents. One of the specific characteristics of intramolecular hydrogen bond is its energy (DEHB ) [7– 9,13,17]. Estimation of DEHB appears to be not a trivial task, as it requires definition of the reference state and estimation of the effects of additional interaction between substituents of inductive, resonance and steric character. The first problem reveals because separate components of the complex are not accessible. One of the three possible conformers with broken hydrogen bridge is in the use of such reference state, mostly often obtained by rotation of OH group on 180°. In [13] it was shown that more proper is the use of the state with the lowest energy, when both OH group and a basic component (dimethylamino group in our case) are rotated out of the bonded conformation. The second problem – the corrections for steric interaction changes upon the chelate ring formation was solved for the group of substi-
75
tuted Mannich bases [13]. The partition of calculated energy of four different conformers was performed and steric effects connected with repulsion of OH and CH2 NðCH3 Þ2 group were estimated on 1–1.5 kcal/mol, respectively. It appeared that adding about 2 kcal/mol to the energy difference between I and IV conformers gives the corrected energy of intramolecular hydrogen bond formation, which can be compared with the energy of intermolecular interaction, and correlates precisely with such characteristics of hydrogen bond as for example N HðOÞ distance or charge density in the critical point of hydrogen bond obtained with the help of AIM method [13]. In this work we attempt to extend previous considerations to a new group of chloro derivatives of Mannich bases which contain the Cl substituent in 3 – position, neighbour to dimethylaminomethyl group. This substituent introduces radical changes in the scheme of interaction. The main effect is strengthening of the intramolecular hydrogen bond. The aim of this work is detailed characterization of this effect, which is similar to found in Schiff bases [18–20], where strong enhancement of the hydrogen bond was found as well as modification of the potential for the proton transfer within the hydrogen bridge. Such steric effects appeared to be a factor allowing access to proton transfer forms so important for photophysic properties of these systems like ground and excited state proton transfer, double fluorescence [21], thermochromism [22], or some biological activity [23]. The importance of steric effects of chloro substituents enhancing the strength of intramolecular hydrogen bond was not discussed in literature so far. We shall perform calculations by MP2/6-31G** method and DFT method at B3LYP/6-31G** and B3LYP/d95** levels for all four conformers shown in Scheme 1 for the compounds presented in Scheme 2. The scheme shows also the labelling of the compounds not containing 3-Cl substituent, used further for comparison. Direct results of energy calculations for these substances were already presented [13]. Besides the analysis of the structural effects of Cl substitution at position 3, on the
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P. Lipkowski et al. / Chemical Physics Letters 370 (2003) 74–82
Scheme 1. Thermodynamic cycle describing the partition of the energy of intermolecular hydrogen bond.
Scheme 2. Definition of labelling system in different chloro substituted of N-(dimethylaminomethyl) phenol.
characteristics of intramolecular hydrogen bond in m-chloro substituted Mannich bases, we aim also to compare the effectiveness of the scheme of hydrogen bond energy estimation, proposed in the previous [13] publication.
2. Computations The quantum chemical calculations were performed with the GA U S S I A N 98 program package [24]. Full geometry optimization was performed at MP2 and B3LYP levels with 6-31G** basis set as well as at B3LYP/d95** level. In [25] it was demonstrated that such basis set gives sufficiently good reproduction of the structure of the compounds of similar size. The AIM [27] analysis has been performed with the AIM2000 code [28], with all default options.
3. Results and discussion 3.1. The structural effects resulting from the formation of intramolecular hydrogen bond Such studies were performed already for a group of four chloroderivatives of N-dimethylaminomethyl phenol (DMAP) and DMAP itself [14]. The aim of these studies was to establish how the structural consequences of the formation of intramolecular hydrogen bond depend on the acidity of the phenol part of intramolecular complex. It was found that increase of DpKa [¼ pKa ðBHþ Þ pKa ðAHÞ, for the AH B hydrogen bond] leads to extension of OH bond length DdðOHÞ and shortening of the CO bond length DdðCOÞ, like in intermolecular interaction. The DdðOHÞ and DdðCOÞ values are the differences between dðOHÞ or dðCOÞ in hydrogen
150.3 150.3 151.0 151.2 151.4 151.1 151.9 151.3 151.4 151.9 152.4 A, B3LYP/6-31G(d,p); B, MP2/6-31G(d,p); C, B3LYP/d95(d,p).
Av. C B
148.9 149.0 149.7 149.8 150.2 149.8 150.6 149.8 150.2 150.7 151.1 149.0 149.2 149.9 150.2 150.3 150.0 150.9 150.3 150.3 151.0 151.5
A Av.
1.3610 1.3587 1.3527 1.3560 1.3503 1.3510 1.3480 1.3541 1.3493 1.3466 1.3451 1.3617 1.3599 1.3536 1.3567 1.3512 1.3522 1.3487 1.3551 1.3506 1.3478 1.3461
C B
1.3638 1.3615 1.3560 1.3592 1.3535 1.3542 1.3516 1.3574 1.3524 1.3501 1.3486 1.3574 1.3548 1.3486 1.3521 1.3463 1.3467 1.3437 1.3498 1.3450 1.3420 1.3405
A Av.
2.7100 2.7008 2.6865 2.6599 2.6777 2.6792 2.6401 2.6461 2.6735 2.6258 2.6190 2.6960 2.6890 2.6760 2.6495 2.6687 2.6696 2.6322 2.6360 2.6646 2.6218 2.6127
C B
2.7099 2.7004 2.6853 2.6633 2.6746 2.6778 2.6403 2.6506 2.6710 2.6229 2.6198 2.7241 2.7130 2.6983 2.6669 2.6897 2.6902 2.6477 2.6517 2.6850 2.6326 2.6245
A Av.
1.8084 1.7971 1.7757 1.7457 1.7624 1.7664 1.7170 1.7294 1.7571 1.7013 1.6885 1.7845 1.7758 1.7560 1.7255 1.7448 1.7476 1.7002 1.7094 1.7395 1.6877 1.6734
C B
1.8142 1.8028 1.7801 1.7564 1.7642 1.7709 1.7234 1.7416 1.7597 1.7072 1.6959
A C
0.9970 0.9984 1.0009 1.0034 1.0030 1.0024 1.0076 1.0053 1.0040 1.0095 1.012 0 1a 1b 1c 2a 2b 2c 2d 3a 3b 4
Av.
0.9892 0.9908 0.9934 0.9945 0.9957 0.9949 0.9995 0.9964 0.9967 0.9975 1.0036 0.9908 0.9925 0.9949 0.9972 0.9969 0.9966 1.0015 0.9992 0.9981 1.0039 1.0059
0.9923 0.9939 0.9964 0.9984 0.9985 0.9980 1.0029 1.0003 0.9996 1.0036 1.0072
B A
1.8264 1.8127 1.7911 1.7552 1.7781 1.7807 1.7274 1.7373 1.7722 1.7091 1.6962
Angle ðOH NÞ rCO rNO rNH rOH
Table 1 The values of the calculated O–H, C–O bond lengths and OH N and O N distances (kcal/mol), for hydrogen-bonded conformer
bonded and open conformers, respectively. The corrections in dðOHÞ and dðCOÞ in the open forms of Mannich bases, resulting from the formation of OH Cl hydrogen bond to o-Cl, atom were made. The calculations of the structure in OH Cl, hydrogen bonded and open forms of related phenols were performed in such a case, cf. [14]. Nevertheless, even after these corrections no linear correlation of DdðOHÞ and DdðCOÞ on DpKa was found [14]. Especially large deviations from these correlations were found, when passing from compounds with 3-Cl substituent to a group of derivatives without this substituent. In this Letter the same calculations were performed for the next six chloro derivatives 2-(N-dimethylaminomethyl) phenol. Table 1 contains the calculated values of distances, which are the most sensitive to modification of hydrogen bond strength, for all further discussed Mannich bases. CompoundsÕ labelling is the same as in Scheme 2. Figs. 1a,b show the dependence of corrected (as described previously, cf. also [14]) increase of dðOHÞ and decrease of dðCOÞ upon the intramolecular hydrogen bond formation as a function of DpKa for all the 11 compounds defined in Scheme 2, averaged over three applied here theoretical methods. One can see a regular increase of these parameters on DpKa . pKa values were taken from [26]. The derivatives with 3-Cl substituents form definitely a separate class of hydrogen bonds, which can be characterized as stronger ones because of larger DdðOHÞ and DdðCOÞ at a particular value of DpKa . This we assigned to steric repulsion of 3-Cl substituent, pushing N-methylaminemethyl group in the direction of OH, and making the hydrogen bond shorter and stronger. Such effects are widely described in orthohydroxy Shiff bases, where external squeezing leads to shortening of the hydrogen bond [18–20]. In a previous publication [13] it was demonstrated that some characteristics of hydrogen bonds can be estimated from the calculation within the AIM model. It was shown that the structural consequences of the hydrogen bond formation well correlate with such parameters like the charge density in bond critical point. It was found that for all 11 compounds discussed in this Letter there exists almost linear, uniform depen-
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149.4 149.5 150.2 150.4 150.6 150.3 151.1 150.5 150.6 151.2 152.0
P. Lipkowski et al. / Chemical Physics Letters 370 (2003) 74–82
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P. Lipkowski et al. / Chemical Physics Letters 370 (2003) 74–82
Fig. 2. Dependence of the charge density qðrÞ at critical point of H N bond on DpKa . j, Points from the compounds with 3-Cl substituent; r, without this substituent.
intramolecular hydrogen bond is stronger in compounds containing the 3-Cl substituents. Both lines have a positive slope in dependency on DpKa . These results well coincide with previously discussed structural effects. Fig. 1. (a) Dependence of the OH bond increment resulting from formation of intramolecular hydrogen bond on DpKa . j, Points from the compounds with 3-Cl substituent; r, without this substituent. (b) Dependence of the CO bond increment resulting from formation of intramolecular hydrogen bond on DpKa . j, Points from the compounds with 3-Cl substituent; r, without this substituent.
dence of charge density in critical point qðrÞ of OH N bond on the dðN HÞ distance. It demonstrates that electron properties of hydrogen bond depend basically on the N H distance despite the fact that there are two different sources for this distance shortening. One is the acid–base interaction, the second is steric shortening of N H. The linear correlation of qðrÞ ¼ a þ bdðN HÞ can be described by the parameters: a ¼ 0:1164, b ¼ 0:2544, R2 ¼ 0:9973. One can use the ÔlocalÕ property of hydrogen bond (charge density in critical point) to characterize its strength (cf. [13,17]). This can be expected as universal measure of the strength of intra- and intermolecular complexes. Fig. 2 presents a dependency of qðrÞ on DpKa . It clearly shows that for series of compounds with m-Cl substituent, the charge in the critical point is substantially higher than that for compounds without 3-Cl substituent. This result can be taken as a direct evidence that
3.2. Energetic consequences of the intramolecular hydrogen bond formation The results of the energy calculations are presented in Table 2. The difference between I and IV forms (DE) is a definition of ÔthermodynamicÕ energy of hydrogen bond, which after some consideration, like zero point energy corrections, can be 0 compared with experimental DHHB values if such are accessible (when open forms are observed in experiment). The obtained values of DE do not correlate with DpKa , which is mainly resulting from the decrease of the energy of reference (IV) state for molecules containing also Cl substituent at 6 position, where intramolecular hydrogen bond of OH Cl type is formed. To correct the energy value on this effect we have performed the calculations for related phenols in the bonded (OH Cl) and open forms 0 (DEðOH ClÞ ). The results are presented in Table 4. Having in disposal the result of calculations of the energy of four different conformers (Table 2) of the compounds able to form intramolecular hydrogen bond one can attempt to estimate the energy of intramolecular hydrogen bond [13]. As it
P. Lipkowski et al. / Chemical Physics Letters 370 (2003) 74–82
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Table 2 The calculated energy differences between particular conformers defined in Scheme 1 (kcal/mol) EI EII
1c 2c 2d 3b 4
EI EIII
EII EIV
EIII EIV
A
B
C
A
B
C
A
B
C
A
B
C
)9.84 )7.29 )10.11 )7.63 )7.65
)9.78 )7.55 )10.01 )7.83 )7.91
)10.59 )7.62 )10.85 )7.96 )7.91
)9.39 )10.05 )9.53 )10.17 )10.37
)9.47 )10.12 )9.55 )10.20 )10.45
)9.89 )10.46 )9.99 )10.54 )10.69
0.37 0.39 0.48 0.49 0.49
0.59 0.62 0.70 0.73 0.73
0.27 0.29 0.39 0.38 0.43
)0.08 3.15 )0.10 3.04 3.20
0.28 3.18 0.25 3.09 3.27
)0.43 3.13 )0.45 2.95 3.20
A, B3LYP/6-31G(d,p); B, MP2/6-31G(d,p); C, B3LYP/d95(d,p).
was mentioned previously no direct measures of DEHB are available experimentally (no open forms are accessible) nor theoretically (one cannot define the reference state with infinitively separated interacting fragments) which is a norm in intermolecular interactions. The calculations proceed according to Scheme 1. Two general possibilities can be accounted. In the case of 3-Cl substituted compounds the scheme does not give a direct measure of the desired property. Additional to all parameters used in the cases of compounds without m-Cl substitution, which are O–H Cl interaction energy DEðO–H ClÞ, steric interaction of –OH ðS2 Þ and –CH2 –NðCH3 Þ2 groups (S1 ) there is also steric interaction of the m-Cl substituent on benzylamine moiety in open conformers III and IV (S3 ). Some changes of S1 and S2 parameters can be expected because of probably different steric interactions also in I and II conformers. Because it is not possible to estimate all these parameters independently, one can use some simplification assuming S1 parameter as for molecules discussed in previous Letter [13]. S1 parameter was changing regularly with the number of chlorine substituents and the average values for the molecules with the same number of chlorine atoms were applied. The value of S1 for 4 was taken from extrapolation in function of the number of substituents, for each calculation method separately. The differences between the energies of II and IV conformers are equal to S1 S3 . In comparison to differences for related molecules without 3-Cl substituent this value is about 1 kcal/mol smaller, what results from compensation by S3 . It allows also direct estimation of S3 which is given in Table 3. This effect of the repulsion of dimethylbenzylamine
group with 3-Cl substituent can be estimated independently, may be more directly (S30 values in Table 3), by calculations of the energy of two conformers of properly Cl-substituted N-benzylamines. Of course then, the difference DEðII IVÞ gives straight the alternative values of S10 , which has the same meaning as S1 , but obtained by using S30 values; S10 ¼ DEðII IVÞ þ S30 . The results are given in Table 3. As one can mention, the results of both ways of S1 and S3 calculations are quite similar, what allows one to believe that those values are estimated with precision not worse than 0:3 kcal/mol, which for such calculations seem pretty good. In further consideration we use the S10 and S30 values as obtained in a more direct way. S2 values were estimated in the previous paper to be 0.9–1.0 kcal/mol, for compounds not containing the 3-Cl substituent. For some of the compounds it was directly known from the difference in energy between III and IV states (cf. Scheme 1) and for others corrected on DEðOH ClÞ0 interactions. In the case of compounds with substituent at 3 position, such calculated values of S20 are considerably lower, from )0.1 to 0.14 kcal/mol (Table 3). This result seems to be very surprising, because one can expect even Table 3 Steric corrections to the energy of intramolecular hydrogen bond (kcal/mol); see text
1c 2c 2d 3b 4
S1
S10
S2
S20
S3
S30
1.26 1.38 1.38 1.48 1.65
1.43 1.63 1.68 1.86 1.94
)0.08 0.88 )0.10 0.88 0.88
)0.08 0.14 )0.10 0.13 0.12
0.85 0.95 0.86 0.95 1.10
1.02 1.25 1.10 1.33 1.40
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P. Lipkowski et al. / Chemical Physics Letters 370 (2003) 74–82
some increase of steric repulsion resulting from introducing the (O)H atom into nearer contact to benzylamine group. This problem will be discussed later on. Having at oneÕs disposal all necessary corrections one can calculate the DEHB values. In the first step one calculates DEðOH NÞ ¼ DE þ DEðOH ClÞ þ S3 ; which is the energy of intramolecular hydrogen bond calculated as a difference in energy between I and IV states, when the last one was corrected on intramolecular hydrogen bond of OH Cl type in the open form of OH Cl type if molecules contain the 6-Cl substituent. In the next step one obtains DEHB from the dependence:
Fig. 3. Dependency of the energy of hydrogen bond, defined as DEðOH NÞ – (see text) on DpKa . j, Points from the compounds with 3-Cl substituent; r, without this substituent.
DEHB ¼ DEðOH NÞ S1 S2 : It can also be calculated using the equation (Scheme 1): DE ¼ DEHB þ S1 þ S2 S3 DEðOH ClÞ: The results are collected in Table 4. They are compared with the results for compounds without 3-Cl substituent in Figs. 3 and 4. From Fig. 3 it is seen that the calculated energy of intramolecular hydrogen bond increases with DpKa . The line for the compounds with 3-Cl substituent is located higher than that for other compounds, which is in full agreement with previously found conclusions based on structural and electronic (qðrÞ) evidences. Surprisingly, the strength of hydrogen bond (Fig. 4) for molecules without 3-Cl substituent appears higher (for a given DpKa value) than that for compounds with 3-Cl. These results contradict to all previously obtained conclusions, showing the strengthening of intramolecular hydrogen bond by steric interactions of 3-Cl substituent. The difference seems to result from problems with the esti-
Fig. 4. Dependency of the calculated energy of intramolecular hydrogen bond in chloro substituted Mannich bases on DpKa . j, Points from the compounds with 3-Cl substituent; r, without this substituent.
mation of S2 correction. If one accepts the values from [13] one obtains the relations between DEHB for both groups of compounds in agreement with all previous reasoning. Estimation of DEðOH NÞ does not require any corrections, but DEðOH ClÞ and S3 . The S1
Table 4 The estimated values of the energy of intramolecular hydrogen bond DEHB and its components (kcal/mol); see text
1c 2c 2d 3b 4
DE
DEðOH ClÞ
DEðOH ClÞ0
DEðOH NÞ
DEðOH NÞ0
DEHB
)9.66 )7.06 )9.78 )7.28 )7.28
– )2.27 – )2.15 )2.35
– )3.01 – )2.90 )3.10
)8.81 )8.38 )8.92 )8.48 )8.53
)8.64 )8.97 )8.68 )8.85 )8.98
)9.99 )10.64 )10.22 )10.83 )11.04
P. Lipkowski et al. / Chemical Physics Letters 370 (2003) 74–82
values are very much resembling these obtained in [13]. S30 values obtained from calculations for benzylamines are of similar order. Only S20 for compounds with 3-Cl substituent are smaller by nearly 1.0 kcal/mol than in [13]. For compounds 1c and 2d they result directly from the difference in energy between III and IV states and possible errors in the DðOH ClÞ estimation do not influence the results. The main difference seems to result from using the different reference states III and IV in both series of chloro substituted DMAP. In compounds with 3-H substituent, the N–C7 –C2 –C3 torsional angle is 25° [14], while in 3Cl substituted compounds it is 75°. The OH group interacts in a different way with the ÔopenÕ N-dimethylmethyl group. A careful comparison of the distances between ÔnonbondingÕ interacting atoms in the vicinity of the chelate ring does not show any distinct differences between two groups of discussed compounds. The difference in energy of III conformers with the geometry obtained for the molecules with 3-H- and 3-Cl substituents is less than 0.1 kcal/mol, however. The only difference seems to be the CH Cl contact, which appears to be shorter in conformer III than in IV on (B3LYP/6-31G**). If it has an attractive 0.06 A character, the cancellation of repulsions (similar like in [13]) and increased H Cl attractions can make the energy difference between III and IV conformers so small, as it follows from calculations. Obtained in this work is the result on estimation where the corrections on steric interactions appeared to be much more complicated than in previously [13] analysed set of compounds. It demonstrates that it is very hard to obtain the systems, where simple portioning of the energy and assuming the additivity of corrections are sufficient to obtain a reliable estimation of the energy of the intramolecular hydrogen bond.
4. Conclusions The results of the energy calculations for four conformations of chloro derivatives of 2-(N-dimethylaminomethyl) phenols at ab initio and DFT levels show that the Cl substituent at position 3 in a
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specific way influences the structure and energy of the intramolecular hydrogen bond. It was demonstrated that steric interaction of this substituent shortens the hydrogen bridge, which makes the hydrogen bond stronger. A more general conclusion goes from performed calculations; independently of the sources of the hydrogen bond shortening – the acid–base attraction or external steric squeezing the charge distribution in H N bond is the same for a system with a given H N distance. Concerning the estimation the energy of intramolecular hydrogen bond, the calculations for the system without 3-Cl substituent seem to give reliable values while the change of the conformation of reference states in 3-Cl-derivatives makes such discussion too complicate. Very delicate changes in geometry introduce so large differences in the energy that it is very hard to obtain an univocal solution. Acknowledgements € AD for financial supThe authors thank the O port within Polish – Austrian exchange program 22/2000, the Cracow Supercomputer Centre and Wroclaw Centre for Networking and Supercomputing. References [1] A. Koll, P. Wolschann, Monatsh. Chem. 130 (1999) 983. [2] T.S. Humble, Ch.J. Halkides, J.D. Keltner, M. Messina, Chem. Phys. Lett. 298 (1998) 90. [3] M. Szafran, J. Chem. Soc. Faraday Trans. 2 (88) (1992) 1261; J. Chem. Soc. Faraday Trans. 90 (1994) 2489. [4] A. Filarowski, A. Koll, Vib. Spectrosc. 17 (1998) 123. [5] K.B. Borisenko, I. Hargittai, J. Mol. Struct. (Theochem) 388 (1996) 107. [6] K.B. Borisenko, C.W. Bock, I. Hargittai, J. Phys. Chem. 100 (1996) 7426. [7] M. Cuma, S. Scheiner, T. Kar, J. Am. Chem. Soc. 120 (1998) 10499. [8] M. Cuma, S. Scheiner, T. Kar, J. Mol. Struct. (Theochem) 467 (1999) 37. [9] M. Fores, S. Scheiner, Chem. Phys. 246 (1999) 65. [10] A. Koll, M. Rospenk, E. Jagodzinska, T. Dziembowska, J. Mol. Struct. 552 (2000) 193. [11] S.M. Melikova, A. Koll, A. Karpfen, P. Wolschann, J. Mol. Struct. 523 (2000) 223.
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