Theoretical Investigation of the Dihydrogen Bond Linking MH 2 with HCCRgF (M ) Zn, Cd; Rg ) Ar, Kr) Mohammad Solimannejad* Quantum Chemistry Group, Department of Chemistry, Arak UniVersity, 38156-879 Arak, Iran Steve Scheiner Department of Chemistry and Biochemistry, Utah State UniVersity, Logan, Utah 84322-0300 ReceiVed: NoVember 3, 2005 An ab initio computational study of the properties of four linear dihydrogen-bonded complexes pairing MH 2 (M d Zn, Cd) with HCCRgF (Rg d Ar, Kr) was undertaken at the MP2/DGDZVP level of theory. The calculated complexation energies of the linear complexes vary between 6.5 kJ/mol for M d Zn to 8.5 kJ/mol for M d Cd. Equilibrium interatomic H‚‚‚H distances are roughly 2.07 Å for all four complexes. The red shifts of the H-C stretching frequency of HCCRgF correlate nicely with the interaction energies. 1. Introduction The subject of dihydrogen bonds has received a great deal of attention lately. 1-6 The dihydrogen bond is an attractive H‚‚‚H interaction, arising from the close approach of a protonic H atom and a hydridic H atom. 7 In a parallel vein, metal hydrides are of considerable importance in chemical synthesis as intermedi- ates in catalytic hydrogenation reactions. Transition-metal atoms react with dihydrogen to produce metal dihydrides or dihydrogen complexes, and these may be trapped in solid matrix samples for infrared spectroscopic study. 8 It has been shown previously that metal hydrides are capable of forming dihydrogen bonds. 9-12 Recently, we have published a series of papers concerning dihydrogen-bonded complexes of metal hydrides with certain rare gas derivatives. 13-15 We turn our attention now to the potential interactions involving the alkynic H of rare-gas systems HC≡CRgF. In the absence of an experimental search for the title complexes up to the present, a theoretical analysis of their properties would appear to be in order. The present work thus reports a detailed examination of the stabilities, electronic structure, and vibrational frequencies of the title complexes for the first time. 2. Computational Details Calculations were performed using the Gaussian03 system of codes. 16 Geometry optimizations and frequency calculations were performed at the MP2 level using DGDZVP basis set. 17 Harmonic vibrational frequency calculations were performed, which confirm the predicted structures as minima in the potential energy surfaces (PESs) of the title complexes The charge distribution has been analyzed by the natural bond orbital (NBO) 18 partitioning scheme at the MP2/DGDZVP level. The counterpoise (CP) method 19 was used to correct basis set superposition error (BSSE) in the calculation of the binding energy. 3. Results and Discussion Association of the linear HCCRgF molecules (Rg d Ar, Kr) with linear MH 2 (M d Zn, Cd) subunits 8 leads to the similarly linear C ∞V complexes HMH‚‚‚HCCRgF, which were calculated to be minima at the MP2/DGDZVP level of theory. Completely optimized geometries for all species are reported in Figure 1. The computed complexation energies of the linear di- hydrogen-bonded complexes are about 6.5 kJ/mol for the Zn complexes, as compared to the larger 8.5 kJ/mol for those that contain Cd, as indicated in Table 1. The nature of the rare gas atom has little effect on this quantity, although there is a slight preference for Kr over Ar. This 0.3 kJ/mol difference, however, is probably within the margin of error of the computational method.While quantitatively different, the interaction energies computed without BSSE correction obey the same trends. The NBO atomic charges displayed in Figure 1 reveal relatively minor changes upon complexation. The process seems to make the “hydridic” H of the HMH, that is, the bridging proton, somewhat more negative by an amount of about 0.05. * Author to whom correspondence should be addressed. E-mail: m-solimannejad@araku.ac.ir. Figure 1. MP2/DGDZVP geometries (Å) and NBO atomic charges (e) of HMH‚‚‚HCCRgF complexes. Results for monomers in italics. 11933 J. Phys. Chem. A 2005, 109, 11933-11935 10.1021/jp0563383 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/03/2005