Comparison of the CH center dot center dot center dot N and CH center dot center dot center dot O interactions involving substituted alkanes

The CH ... N interaction between FnH3-nCH (proton donor) and NH3, CH3NH2, a nd CH2NH (acceptor) is examined by ab initio calculations and compared with the comparable CH ... O systems. The interaction is strengthened by 1.2 kc al/mol with each F added to the donor, slightly larger than the 1.0 kcal/mo l increment for CH ... O. Like CH ... O, the CH ... N interaction behaves l ike a conventional H-bond in terms of electron density shifts and the magni tudes of the various components of the interaction energy. The CH ... N int eractions are also like CH ... O in that they both die off more slowly as t he H-bonds are stretched, in comparison to corresponding conventional H-bon ds. Contrary to the behavior of the OH donor, the CH bond contracts and und ergoes a blue shift upon forming a complex with either N or O acceptor, alt hough these changes are somewhat smaller for CH ... N and not as systematic . The differing trends are attributed to the level of cancellation between opposing forces, and not due to any fundamental difference between the two types of interaction. (C) 2000 Elsevier Science B.V. All rights reserved.