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Notes: A potential function for describing interactions between formamide molecules is proposed. The function was developed from ab initio computed molecular properties and intermolecular perturbation theory (IMPT) calculations for formamide dimer involving the 6-311G** basis set. It consists of an electrostatic term that is a function of multipoles distributed over the atoms, an exponential repulsion term obtained by fitting results for the dimer, an induction contribution that is a function of atomic polarizabilities, and a dispersion term based on a London expression that is also dependent on atomic polarizabilities. The results obtained by applying the proposed function to formamide dimer are similar to those found at the MP2/6-311G** level; the function allows one to identify five structures corresponding to as many energy minima. Application of the function to larger clusters revealed that the most attractive minima correspond to planar structures, the most common structural pattern among which is that of the global minimum for the dimer. Based on cooperativeness data for the larger clusters, chained structures seemingly form stronger hydrogen bonds due to increased cooperativeness in interactions between molecules, which may account for the tendency of condensed phases of formamide to adopt open structures. © 1999 American Institute of Physics.

Notes: Methylamine clusters consisting of up to four molecules were studied by employing Hartree–Fock, density functional theory, and Moller–Plesset calculations with the 6-31+G* basis set. The dimer was found to exhibit two minima with similar interaction energies (−13 kJ/mol) and involving a hydrogen bond. The dipole moment for the dimer differs by up to 20% from the vector addition of the dipole moments for the individual molecules by effect of the interaction; also, the N–H bond distance in the group involved in the hydrogen bond is lengthened by up to 0.006 Å as a result. The minima identified for the trimer and tetramer possess cyclic structures that differ in the position of the methyl groups with respect to the plane containing the hydrogen bonds. The contribution of nonadditivity to the interaction in these structures is quite significant (12%–18% of the overall interaction energy). N–H distances in the donor molecule are longer than in the dimer and increase with increasing cluster size. Likewise, the hydrogen bonding energy increases with cluster size. These results expose the significance of cooperative phenomena in the interactions between methylamine molecules. The computations predict slight shifts in the C–N stretching frequencies, which are consistent with their experimental counterparts. The N–H stretching and the NH2 wagging modes undergo large shifts, with a magnitude that increases as the size of the cluster grows. © 2000 American Institute of Physics.

Notes: Potential functions for describing interactions between cyanoacetylene molecules based on ab initio determined molecular properties and IMPT calculations are proposed. Electrostatic interactions are described by a multipole expansion on atoms and midbond points; dispersion is expressed by a London-type function of atomic polarizabilities and induction is considered via a series of polarizabilities distributed over the atoms. The repulsion contribution was determined by using a test-particle model involving a helium atom as probe particle. Two functions based on two basis sets of different size, viz. 6-311G** and 5S4P2D/3S2P, were used. Cyanoacetylene dimer exhibits two minima corresponding to a linear and an antiparallel configuration, respectively. The proposed functions accurately reproduce the characteristics of the dimer minima as derived from ab initio calculations at the Møller–Plesset (MP2) level. In addition, they can describe cooperativeness in larger clusters; specifically, the dipole moment and interaction energy per molecule increase with increasing number of constituent units in the cluster. The behavior observed is similar to previously reported findings for HCN clusters. © 1998 American Institute of Physics.

Notes: We developed potential functions for (HCN)2 and used them to study larger aggregates. The functions are based on ab initio perturbation calculations for the dimer and on molecular properties also estimated from ab initio calculations involving two different basis sets. The overall energy is resolved into electrostatic, repulsion, induction, and dispersion components. The electrostatic contribution is represented by a multipole expansion at several sites. The repulsion function is of the exponential type and includes atomic anisotropy. The dispersion is described by means of a London-type formula. Finally, the induction contribution is expressed in terms on polarizabilities distributed among the atoms. The proposed functions accurately reproduce the characteristics of the HCN dimer and the two minimum-energy forms of the trimer. Also, they predict various properties (e.g., the dependence of the dipole moment and interaction energy on cluster size) of larger aggregates. © 1998 American Institute of Physics.

Notes: The ionic dissociation of H–X acids (X=F, Cl, Br, I) in water was examined by conducting a theoretical study on the properties of the clusters formed by the acids with up to five water molecules: X–H(H2O)n (n=1–5). Calculations were done using the DFT/B3LYP and MP2 methods in conjunction with the TZVP basis set and allowed the identification of several minima on the potential surfaces for the clusters. Based on the results, the MP2 method predicts a lower tendency to ionization than does the DFT/B3LYP method; however, both methods provide similar results. The dissociation characteristics of the acids were examined in terms of various parameters including the lengths of the bonds involved in the proton transfer and the frequencies associated with the X–H and O–H stretching modes in the bonds taking part in the proton transfer. The successive incorporation of water molecules to the cluster was found to lengthen X–H distances and simultaneously decrease O(centered ellipsis)H distances. In addition, the X–H stretching frequency underwent a marked redshift; the signal disappeared in the ionized structures, at the expense of a new series of bands around 2800 cm−1 due to stretching vibrations of the O–H bond in the H3O+ ion. Hydrogen fluoride failed to dissociate in the clusters considered; in fact, while some structures were ionized, they were not the most stable configurations for the corresponding clusters. In HCl and HBr, the dissociated structure was the most stable in the clusters of four or more water molecules (n=4–5); however, HBr exhibited a stronger tendency to dissociating above n=3. Finally, HI exhibited dissociation at n〉2.© 2002 American Institute of Physics.

Notes: In this work, clusters consisting of two and three formamide or thioformamide molecules were subjected to ab initio and density functional theory calculations using the aug-cc-pvdz/cc-pvdz basis set. Formamide and thioformamide dimers were both found to exhibit five different minima on their potential surfaces involving hydrogen bonds of the N–H(centered ellipsis)X(Double Bond)C or C–H(centered ellipsis)X(Double Bond)C (X(Double Bond)O, S) type. The most stable structure in both cases is a cyclic configuration of C2h symmetry involving two identical N–H(centered ellipsis)X(Double Bond)C bonds. The interaction energy for such a structure is −60 and −48 kJ/mol for formamide and thioformamide, respectively. Based on the calculations, each N–H(centered ellipsis)X(Double Bond)C bond contributes −30 kJ/mol to it in formamide and −24 kJ/mol in thioformamide. On the other hand, each N–H(centered ellipsis)X(Double Bond)C bond contributes −9.7 kJ/mol in formamide and −11.7 kJ/mol in thioformamide. The interaction causes appreciable distortion in the molecules, particularly in the N–H groups involved in a hydrogen bond, which are lengthened by up to 0.019 and 0.013 Å in formamide and thioformamide, respectively. The trimer structures identified on the potential surfaces of formamide and thioformamide are cyclic configurations capable of establishing 3 or 4 hydrogen bonds. While formamide tends to adopt planar configurations (the most stable of which possesses an interaction energy of −105 kJ/mol), thioformamide forms preferentially nonplanar structures (the most stable being a nonplanar cyclic configuration with an interaction energy of −88 kJ/mol). The contribution of nonadditive pairwise terms is not particularly significant in either compound, which suggests the absence of substantial cooperative phenomena in the trimers. However, this contribution is crucial with a view to determining the stability sequence for the trimers, where the most stable structures result from the contribution of nonadditive pairwise terms (up to 15% of the overall interaction energy for the most stable thioformamide trimer). The interaction shifts the frequencies of modes closely involved in it. Thus, the N–H symmetric stretching frequency is redshifted by more than 300 cm−1 and the NH2 wagging frequency is blueshifted to a similar extent. As a rule, frequency shifts are less marked in the thioformamide clusters; both substances, however, exhibit identical trends. © 2002 American Institute of Physics.

Notes: Clusters consisting of two and three dimethylamine molecules were studied by using the HF, DFT/B3LYP, and MP2 ab initio methods in conjunction with the 6-31+G* and aug-cc-pvdz/cc-pvdz basis sets. Three different minima were located for the dimer, two of which form a N–H(centered ellipsis)N hydrogen bond and present similar interaction energies. The most stable structure of the dimer possesses Cs symmetry and an interaction energy of −15.6 kJ/mol. The least stable minimum has an interaction energy of −7.9 kJ/mol and exhibits no N–H(centered ellipsis)N hydrogen bonds (the interaction is established via the methyl hydrogen atoms). In all the structures, electron correlation exhibits a significant contribution (more than 40% of the overall energy). Only cyclic structures were considered for the trimer, the most stable of which possesses an interaction energy of −43.9 kJ/mol. The dipole moments for the dimer differ by up to 30% from the vector addition of the molecular dipoles (in the trimer minima, this difference amounts to 40%); also, N–H distances are lengthened by effect of the interaction (by up to 0.004 Å in the dimer and 0.009 Å in the trimer), which suggests the presence of cooperative phenomena. Nonadditive terms contribute about 12% of the overall interaction energy, the contribution being primarily of the inductive type. Calculations predict significant red shifts in the vibrational frequency of the N–H group when it takes part in the formation of a hydrogen bond. Similarly, the N–H wagging frequency undergoes a blue shift with hydrogen bonding. © 2000 American Institute of Physics.