Intermolecular Interactions :

1. Exploring The Lower Limit in Hydrogen Bonds: Analysis of Weak C-H…O and C-H… Interactions in Substituted Coumarins From Charge Density Analysis P. Munshi and Tayur N. Guru Row munshi@sscu.iisc.ernet.in  ssctng@sscu.iisc.ernet.in Solid State and Structural Chemistry Unit, Indian Institute of Science, Bagalore-560012, INDIA Introduction: Weak C-H…O interactions have been extensively reviewed in several articles and books.1 However Steiner points out that theoretical understanding of C-H…O interaction (and weak hydrogen bonding in general) is still unsatisfactory and experimentalists continuously suffer from a lack of theoretical backing. Characterization of C-H…O hydrogen bonds2 on the basis of the theoretical charge density serves as a source of information to study energetics. Mallinson et. al.3 have found a ‘Morse-like’ dependence of the Laplacian of electron density on the length of interaction line, which suggests a method to differentiate ionic and covalent bond characters based on the strength of the interaction. Several general relationships appear which allows for “quantitative crystal engineering” which would fine tune intermolecular interactions to match with the following criteria:2 Figure 1 1. Consistent topology (BCP, BP, IAS) for each hydrogen bond 2. A b value at the BCP lies within the range [0.015, 0.3] eÅ-3 3. A 2b value at the BCP lies within the range [3.0, 0.5] eÅ-5 4. Mutual penetration of hydrogen and acceptor atom [rD+ rA] BCP = Bond Critical point BP = Bond Path IAS = Inter Atomic Surface Figure 2 Local energy densities: P. E. K. E. Intermolecular Interactions: We present the nature of C-H…O and C-H… interactions based on the experimental (via XD4) and theoretical (via CRYSTAL035) charge density analysis on a series of compounds (Scheme 1), 2H-chromen-2-one (coumarin),2H-thiochromen-2-one (1-thiocoumarin)and3-acetyl-2H-1-benzopyran-2-one (3-acetylcoummarin) and evaluate them based on Koch and Popelier’s first four criteria.2 The C-H… interactions considered for this study are limited to the aromatic regions only. Figure 3 Observations: Figure 1 – 4 shows the relation of b, 2b, G(rCP), V(rCP) and [rD+ rA] with the length of the interaction line Rij. Significantly, the shaded region between the Rij values of 2.75 – 2.85 Å contain representations from both C-H…O and C-H… contacts, which Figure 4 Scheme 1 we believe is a “region of overlap” between the hydrogen-bond and an interaction. This could be regarded as the lower limit for the formation of a hydrogen bond. Conclusion: The lacuna of the identification of a lower limit for the hydrogen bond formation has been addressed in our analysis. We believe that for the first time there is an experimental indication that at a critical distance X(donor)-H…A(acceptor) interaction is switched from “hydrogen bond” to “van der Waals” type. These yardsticks form the tools of “quantitative crystal engineering”, which allow for the identification of lower limit of a hydrogen bond. 1. (a) Steiner, T. Crystallogr. Rev.2003, 9(2-3), 177; (b) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bonds: In Structural Chemistry and Biology, Oxford University Press: Oxford, New York. 1999; (c) Desiraju, G. R. Acc. Chem. Res.1991, 290. 2. (a) Koch, U.; Popelier, P. L. A. J. Phys. Chem. 1995. 99, 9747; (b) Popelier, P. L. A. AIM –An Introduction, Prentice Hall, UK, 2000. 3. Mallinson, P.R., Smith, G.T., Wilson, C.C., Grech, E., & Wozniak, K. J. Am. Chem. Soc. 2003,125,4259. 4. Koritsanszky,T.S., Richter, T., Macchi,P., Volkov,A., Gatti,C., Howard,S., Mallinson,P.R., Farrugia,L., Su,Z., & Hansen,N.K. 2003, XD. 5. Saunders, V. R., Dovesi, R., Roetti, C., Causa, M., Harrison, N. M., Orlando, R., Zicovich-Wilson, C. M. 2003,CRYSTAL03. Acknowledgement P. M. thanks the CSIR, India for SRF. We are grateful to Prof. P. R. Mallinson for useful suggestions. References