Since postulated in 1920, [1] hydrogen bonding plays an important role in chemistry. [2] This profound interaction contributes much to the physical and chemical properties of substances ranging from small and simple water molecules to large and complex proteins. The definition and model description of H bonds are an active topic of many studies. After years of controversy, the original concept of hydrogen bonding in XÀ H•••Y, where X is a highly electronegative atom such as N, O or F, was extended to include CÀH•••Y. [3] The concept was further extended to include increased CÀH stretching frequencies, and a mechanism of blue-shifting H bonds, which is hotly debated, was developed. [4] Another attention-attracting H bond is the "dihydrogen bond", XÀH•••HÀY, with a metal element or B, Ga and Si as Y atom. [5,6] All the H bonds mentioned above were verified in many substances but so far there is no report of a system in which they appear concurrently. Surprisingly, the three H bond types were found in our study to coexist in a very small system of gaseous valine conformers.
The conformational space of valine was investigated through B3 LYP/6-311
) geometry optimization of the 108 trial structures generated by all combinations of singlebond rotamers [7] with the Gaussian 98 program package. [8] Compared with the previous results, [7,9] five new conformers were found (see Supporting Information for energetic and structural information of all conformers). The stabilities of the five new structures were verified by harmonic frequency analysis and MP2/6-311
p) geometry optimization. In particular, one new conformer, denoted as conformer A here, has a relatively low energy, and its structure is shown in Figure 1. The distances of the three atom pairs indicated in Figure 1 are respectively 1.94 (N•••H), 2.55 (O•••H) and 2.00 (H•••H), and all of them meet the geometric criteria of a hydrogen bond.
Clearly, the pair H•••H bond has the appearance of a dihydrogen bond, XÀH•••HÀY, and the angle between donor and acceptor (aNÀHÀH = 1298) is larger than the angle between acceptor and donor (aCÀHÀ-H = 1108) which coincides with the rule found for dihydrogen bonds. [5] As the C atom is more electronegative than the H atom, this type of dihydrogen bond is