Structural Chemistry, Vol. 16, No. 3, June 2005 ( C  2005) DOI: 10.1007/s11224-005-4461-x The Role of Intramolecular Hydrogen Bonds vs. Other Weak Interactions on the Conformation of Hyponitrous Acid and Its Mono- and Dithio-Derivatives Jenny Zevallos, 1,3 Alejandro Toro-Labb´ e, 1 Otilia M´ o, 2 and Manuel Y ´ a˜ nez 2 Received June 3, 2004; revised July 8, 2004; accepted July 8, 2004 High level ab initio and density functional theory calculations have been carried out to investigate the relative stability of the different conformers of hyponitrous acid and its mono- and dithio- derivatives. Geometries and vibrational frequencies were obtained at the B3LYP/6-311+G(d,p) level and final energies through B3LYP/6-311++G(3df,2pd) single point calculations. The reliability of this theoretical scheme has been assessed by comparing these DFT results with those obtained at the G3 level of theory, for some suitable cases. The cis conformers of hyponitrous acid and its mono- and dithio-derivatives are systematically more stable than the trans ones because in the cis conformation a dative interaction between the nitrogen-lone pairs and the σ ∗ NX antibonding orbital is significantly favored. Quite interestingly, in general, the conformers presenting an intramolecular hydrogen bond (IHB) are not the global minima of the corresponding potential energy surfaces and only for hyponitrous acid the conformer with a OH ··· O IHB is slightly more stable than the cis conformer without IHB. The low stability of the tautomers with IHB is closely related with another weak intramolecular interaction which involves the lone-pairs of the chalcogen atoms and the π ∗ NN antibondig orbital, and which is significantly perturbed when the IHB is formed. KEY WORDS: Intramolecular hydrogen bonds; weak interactions; hyponitrous acid; thio-derivatives; ab initio calculations. INTRODUCTION Intramolecular hydrogen bonds (IHB) play an im- portant role in chemistry [1, 2], because, in general, they are responsible for the enhanced stability of chelated structures, in which two heteroatoms of the same com- pound are connected through a hydrogen atom. More importantly, the existence of an IHB may have a non- negligible effect on the intrinsic basicity or acidity of the system. As it has been shown, for tropolone [3], β -chalcogenvinylaldehydes [4], resorcinol [5], and bis(tetramethylguanidine) superbases and related systems [6, 7] the perturbation of the IHB upon protonation, or 1 Laboratorio de Qu´ ımica Te´ orica Computacional (QTC), Facultad de Qu´ ımica, Pontificia Universidad Cat´ olica de Chile, Santiago, Chile. 2 Departamento de Qu´ ımica, C-9. Universidad Aut´ onoma de Madrid. Cantoblanco, 28049-Madrid, Spain. 3 To whom correspondence should be addressed; e-mail: pclaro@dq. ua.pt its disappearance in deprotonation processes, has a strong effect on the gas-phase basicity and acidity of these com- pounds. Similarly, the formation of an intramolecular hy- drogen bond in the protonated species is responsible for a significant enhancement of the intrinsic basicity of some acid anhydrides [8]. Finally, IHB’s are crucial in many ultra-fast hydrogen transfer phenomena through the use of pulsed lasers [9–12]. In some specific cases, other weak interactions, such as chalcogen–chalcogen interactions [13] in β - chalcogenvinylaldehydes, compete in strength with IHB’s [14]. As a matter of fact, in HC( O) CH CH TeH, the O ··· Te H intramolecular interaction, that can be viewed as dative bond from the oxygen lone-pair toward the σ ∗ Te−H antibonding orbital [14], is much more stabilizing than the corresponding O H ··· Te or O ··· H Te IHB’s. In this paper we aim at analyzing, whether hyponi- trous acid and its mono- and dithio-derivatives are sta- bilized through the formation of X H ··· Y (X O, S; Y O, S) IHB’s or, on the contrary, the conformer with a 295 1040-0400/05/0600-0295/0 C  2005 Springer Science+Business Media, Inc.