Features of the CH⋅⋅⋅N Weak Hydrogen Bond and Internal Dynamics in Pyridine–CHF₃

Abstract

The structural and energetic features of the C À H···N interaction and the internal dynamics of the pyridine-trifluoromethane molecular complex in its normal, 15 N and 13 C species are here reported. They have been obtained from analysis of the pure rotational spectra of the complex generated in a supersonic expansion.Weak hydrogen bonds (WHB) are a major topic in hydrogen-bond research. The energies of these interactions lie within a few kJ mol À1 and approach those of van der Waals forces, but have the same directional properties of "classical" hydrogen bonds.[1] This behavior was recently shown for the CÀH···O, CÀH···FÀC, CÀH···S, and CÀH···p linkages by investigating the rotational spectra of several hydrogenbonded molecular complexes generated in supersonic jets. [2][3][4] With this technique, precise information on the energetics and on the structural and dynamical aspects of such interactions is obtained in an environment free from the intermolecular interactions that take place in condensed media.Investigation of the rotational spectrum of benzene-trifluoromethane has shown that this complex is a symmetric top, with the two moieties held together through a C À H···p interaction, [4] and thus provided information on this kind of WHB. When replacing benzene with pyridine, two sites of high electronic density become available in the ring, so the two adducts shown in Figure 1 could be plausibly formed for the pyridine-trifluoromethane (Py-CHF 3 ) complex.Conformation I resembles the shape of the complex formed between benzene and trifluoromethane, with a CÀ H···p linkage, [4] while species II is similar to the complexes that the three diazines form with water.[5] Before the search of the rotational spectrum was started, full geometry optimizations of the monomers and dimers were carried out at the MP2/6-311 + + G** level of theory by using the Gaussian 03 suite of programs.[6] All energies were corrected for basis set superposition error (BSSE) by using the counterpoise procedure.[7] The calculations suggest that form I is not a minimum, while for species II, we obtained the estimated values of the spectroscopic constants listed in the second column of Table 1. These values were used as a guide in the assignment of the spectrum of species II.In Figure 2 we give the atom numbering, the principal axes system, and the parameters of the internal rotation of the CHF 3 group.Following these predictions, two frequency scans of a few hundred MHz each were carried out in the region of the J = 8-7 and 9-8 m a -type R bands. The corresponding K À1 = 0, 1 transitions were easily assigned. Then, seven m a -type R bands, typical of a near-prolate top, evenly spaced by the B + C value and with J in the range from 7 to 15 were measured on the whole, with K À1 ranging from 0 to 3. Most of [a] Dr.

The Conformational Landscape of Nicotinoids: Solving the Conformational Disparity of Anabasine

“…The N···H distances of 2.686 (syn) or 2.667 (anti) are within the 2.5-2.7 range for these weak interactions. [13,14,15] The observed conformations of anabasine can be rationalized in terms of the structure of related compounds. The preference of the chaired piperidine moiety for a N-H equatorial conformation is well established, as observed for the isolated molecule in piperidine, [16] tropinone, [17] and 1methyl-4-piperidinone.…”

Probing the CH⋅⋅⋅π Weak Hydrogen Bond in Anesthetic Binding: The Sevoflurane–Benzene Cluster

“…Weak hydrogen bonds are characterized by very low interaction energies (< 20 kJ mol À1 ), making these forces especially sensitive to modulation and cooperativity. Model molecular clusters formed by haloalkanes and small organic molecules reveal how C À H···O, [1] C À H···S, [2] C À H···N, [3] or C À H···F À C [4] interactions tend to associate to maximize the number and strength of nonbonding forces, as illustrated by the nine simultaneous CÀH···F contacts in the cage structure of the difluoromethane trimer. [5] Much less structural information is available on the weaker (dispersion-dominated) C À H···p interaction, despite its significance in organic, organometallic, and biological chemistry.…”

The Cage Structure of IndanCHF3 is Based on the Cooperative Effects of CH⋅⋅⋅π and CH⋅⋅⋅F Weak Hydrogen Bonds

“…Indeed, CÀH···O, CÀH···F-C, CÀH···N, CÀH···S, and CÀH···p linkages were characterizedb yi nvestigating the rotational spectra of molecular complexes of CHF 3 with several organic molecules in supersonic jets. [4][5][6][7][8][9][10][11][12] Otheri nteresting features of the spectra of this kind of complex are due to the internal dynamics of CHF 3 , that is the internal rotation aroundits C 3 symmetry axis.…”

“…Large splittings were measured in the case of ArÀ CHF 3 , [13] whereas smalls plittings were observed for the complexeso fH CF 3 with CH 3 F, [7] C 2 H 4 O, [8] thiirane, [9] and pyridine. [10] In complexes with heavier molecules,s uch as cyclobutanone [11] and 1,4-dioxane, [12] and with the CF 3 group linked to the partner molecule by two CÀH···F interactions, the V 3 barrierw as too high to allow the observation of tunnelings plittings.…”