Low energy collisions of CN(X2Σ+) with He in magnetic fields

A theoretical investigation of the He-CN((2)Σ(+)) complex is presented. We perform ab initio calculations of the interaction potential energy surface and carry out accurate calculations of bound energy levels of the complex including the molecular fine structure. We find the potential has a shallow minimum and supports seven and nine bound levels in complex with (3)He and (4)He, respectively. Based on the potential the quantum scattering calculation is then implemented for elastic and inelastic cross sections of the magnetically trappable low-field-seeking state of CN((2)Σ(+)) in collision with (3)He atom. The cold collision properties and the influence of the external magnetic field as well as the effect of the uncertainty of interaction potential on the collisionally induced Zeeman relaxation are explored and discussed in detail. The ratios of elastic to inelastic cross sections are large over a wide range of collision energy, magnetic field, and scaling factor of the potential, suggesting helium buffer gas loading and cooling of CN in a magnetic trap is a good prospect.     

On the properties of X···N noncovalent interactions for first-, second-, and third-row X atoms

In addition to a structure with a PH···N H-bond, a second complex of greater stability is formed when the PH(3) is rotated such that its P-H bond is pointing away from the approaching N lone pair of NH(3). Quantum calculations are applied to examine whether such a complex is characteristic only of P, or may occur as well for other atoms of the first, second, or third rows of the periodic table. The molecules PH(3), H(2)S, HCl, AsH(3), and NH(3) are all paired with NH(3) as electron donor. While NH(3) will not engage in an N···N attraction, all the others do form a X···N complex. The energetics, geometries, and other properties of these complexes are relatively insensitive to the nature of the X atom. This uniformity contrasts sharply with the H-bonded XH···N complexes where a strong sensitivity to X is observed. The three-dimensional nature of the electrostatic potential, in conjunction with the striving for a linear H-X···N orientation that maximizes charge transfer, serves as an excellent tool in understanding both the shape of the potential energy surface and the proclivity to engage in a X···N interaction.     

Quantum theory of molecular collisions in a magnetic field: efficient calculations based on the total angular momentum representation

An efficient method is presented for rigorous quantum calculations of atom-molecule and molecule-molecule collisions in a magnetic field. The method is based on the expansion of the wave function of the collision complex in basis functions with well-defined total angular momentum in the body-fixed coordinate frame. We outline the general theory of the method for collisions of diatomic molecules in the (2)Σ and (3)Σ electronic states with structureless atoms and with unlike (2)Σ and (3)Σ molecules. The cross sections for elastic scattering and Zeeman relaxation in low-temperature collisions of CaH((2)Σ(+)) and NH((3)Σ(-)) molecules with (3)He atoms converge quickly with respect to the number of total angular momentum states included in the basis set, leading to a dramatic (>10-fold) enhancement in computational efficiency compared to the previously used methods [A. Volpi and J. L. Bohn, Phys. Rev. A 65, 052712 (2002); R. V. Krems and A. Dalgarno, J. Chem. Phys. 120, 2296 (2004)]. Our approach is thus well suited for theoretical studies of strongly anisotropic molecular collisions in the presence of external electromagnetic fields.     

The interaction of formohydroxamic acid with carbon monoxide: FTIR matrix isolation and quantum chemistry studies

Argon matrix infrared spectra of the complexes formed between formohydroxamic acid (HCONHOH) and carbon monoxide have been recorded. The experimental results indicate formation of three isomeric complexes. In two complexes the NH or OH groups of formohydroxamic acid are attached to the carbon atom of the CO molecule and in the third complex the NH group interacts with the oxygen atom of CO. The formohydroxamic acid complex with two CO molecules is also trapped in the matrix. One of the two CO molecules interacts with the NH group and the second one with the OH group of HCONHOH, in both cases the site of interaction is the carbon atom of CO. Theoretical studies of the structure and spectral characteristics of the complexes were carried out on the DFT(B3LYP)/6-311++G(2d,2p) level. The calculated vibrational frequencies for the complexes present in the matrices are in good agreement with the experimental data. The calculations show also an additional potential energy minimum corresponding to the complex in which the OH group of formohydroxamic acid is attached to the oxygen atom of carbon monoxide.     

Dipole moment and rovibrational intensities in the electronic ground state of NH3: bridging the gap between ab initio theory and spectroscopic experiment

We report theoretical values for the transition moments of an extensive set of vibrational bands in the electronic ground state of (14)NH(3). For selected bands, we have further made detailed simulations of the rotational structure. The calculations are carried out by means of recently developed computational procedures for describing the nuclear motion and are based on a high-level ab initio potential energy surface, and high-level dipole moment surfaces, for the electronic ground state of NH(3). The reported theoretical intensity values are compared to, and found to agree very well with, corresponding experimental results. It is believed that the computational method, in conjunction with high-quality ab initio potential energy and dipole moment surfaces, can simulate rotation-vibration spectra of XY(3) pyramidal molecules prior to observation with sufficient accuracy to facilitate the observation of these spectra. By degrading the accuracy of selected elements of the calculations, we have also investigated the influence of customary approximations on the computed intensity values.     

Interaction of dihydrogen with small and light molecules

Second-order M?ller-Plesset (MP2) calculations (using the approximate resolution of the identity, RI-MP2), explicitly correlated MP2 (MP2-R12) calculations, and coupled-cluster calculations including all single and double excitations with a perturbative estimate of triple excitations [CCSD(T)] are performed to study the interaction of molecular hydrogen with the small molecules HF, H2O, NH3, and LiOH. Different adsorption positions are studied. In the cases of H2O and NH3, the most favorable configuration places H2 in an end-on fashion on the O or N atom, respectively. In the cases of HF and LiOH, the H2 molecule takes a side-on position on the H atom of HF or the Li atom. With respect to MP2 calculations in a triple-zeta basis, both the enlargement of the basis set and the extension of the correlation treatment (CCSD(T) vs MP2) increase the interaction energy. The basis set limit CCSD(T) estimates of the interaction energy of H2 with the HF, H2O, NH3, and LiOH molecules amount to 4.40, 2.67, 3.02, and 10.74 kJ mol-1, respectively. The interaction energy for the simultaneous interaction of H2 with two LiOH molecules does not significantly exceed the value obtained for the interaction with a single LiOH molecule. Furthermore, the interaction energies (by MP2) of H2 with glycine, the glycine dimer, and imidazolium chloride amount to 2.78, 5.00, and 6.30 kJ mol-1, respectively.     

Solvation of yttrium with ammonia revisited. Di-amide formation in the reaction of yttrium with ammonia

The reactivity of yttrium atoms toward ammonia is revisited using expanded density functional theory calculations. The new results reveal that absorption of NH3 on YNH is dissociative to form Y(NH2)2.The di-amide species can adsorb further NH3 molecules molecularly to form Y(NH2)2NH3 and Y(NH2)2(NH3)2. The calculations aimed to reveal the detail of the potential energy curves between the imide and the di-amide forms. The Y(NH2)2(NH3)x species are more stable than those of YNH(NH3)x by more than 20 kcal/mol.     

Intermolecular potential to represent collisions of protonated peptide ions with fluorinated alkane surfaces

The MP2/6-311++G(2df,2pd) level of theory was used to calculate intermolecular potential curves between CF(4), as a model for the C and F atoms of a fluorinated alkane surface, and CH(4), NH(3), NH(4)(+), H(2)CO, and H(2)O as models for different types of atoms and functional groups comprising protonated peptide ions. This level of theory was tested by comparisons with the MP2/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ theories. Explicit-atom (EA) analytic potential energy functions were then derived by fitting these potential energy curves with two-body potentials between the atoms of the two interacting molecules. An intermolecular potential for the interaction of a protonated peptide ion with a fluorinated alkane surface may be constructed from these two-body potentials. Intermolecular potentials, for which CF(4) is treated as a united atom (UA), were developed by isotropically averaging the CF(4) orientation for each of the EA potential energy curves. The intermolecular potential energy curves calculated for CF(4) are compared with curves calculated previously for CH(4) interacting with the same molecules, to consider the relative efficiency of energy transfer for protonated peptide ion collisions with hydrogenated and fluorinated alkane surfaces.     

Photoinduced radical processes on the spinel (MgAl2O4) surface involving methane, ammonia, and methane/ammonia

The present study explored photoinduced radical processes caused by interaction of CH(4) and NH(3) with a photoexcited surface of a complex metal oxide: magnesium-aluminum spinel (MgAl(2)O(4); MAS). UV irradiation of MAS in vacuo yielded V-type color centers as evidenced by the 360 nm band in difference diffuse reflectance spectra. Interaction of these H-bearing molecules with photogenerated surface-active hole states (O(S)(-)?) yielded radical species which on recombination produced more complex molecules (including heteroatomic species) relative to the initial molecules. For the MAS/CH(4) system, photoinduced dissociative adsorption of CH(4) on surface-active hole centers produced ?CH(3) radicals that recombined to yield CH(3)CH(3). For MAS/NH(3), a similar dissociative adsorption process led to formation of ?NH(2) radicals with formation of NH(2)NH(2) as an intermediate product; continued UV irradiation ultimately yielded N(2). For the mixed MAS/CH(4)/NH(3) system, however, interaction of adsorbed NH(3) and CH(4) on the UV-activated surface of MAS yielded ?NH(2) and ?CH(3) radicals, respectively, which produced CH(3)-NH(2) followed by loss of the remaining hydrogens to form a surface-adsorbed cyanide, CN(S), species. Recombination of photochemically produced radicals released sufficient energy to re-excite the solid spinel, generating new surface-active sites and a flash luminescence (emission decay time at 520 nm, τ ~ 6 s for the MAS/NH(3) case) referred to as the PhICL effect.     

The spin coupling in the diiron complex [Fe2(hpdta)(H2O)3Cl

Density functional, multireference configuration interaction, and modified valence configuration interaction calculations are used to investigate the electronic structure and spin coupling of the dinuclear [Fe(2)(hpdta)(H(2)O)(3)Cl] complex (H(5)hpdta = Hydroxypropane-1,3-diamine-N,N,N',N'-tetraacetic acid). The density functional calculations give evidence of both, states with local high-spin iron centres and states with local low-spin iron centres, the relative energy of which strongly depends on the functional. The splitting of states due to the spin coupling between the high-spin iron centres varies by more than a factor of two for different functionals. In an attempt to study to what extent it is possible to undertake configuration interaction calculations on such binuclear compounds, multireference configuration interaction calculations are performed on a [Fe(2)(OH)(5)(H(2)O)(3)(NH(3))(2)Cl] model complex. The results show that, when correlating only the ten iron 3d orbitals and the four valence orbitals of the bridging OH group, the calculated splitting is still by a factor of about 3 smaller than the value for the splitting inferred from magnetic susceptibility measurements. Modified valence configuration interaction calculations are performed to approximately take into account the influence of orbital relaxation effects of all occupied orbitals in the excited configurations. The exchange splitting is significantly increased, but still smaller than the experimental value.     

Charge transfer in the electron donor-acceptor complex BH3NH3

As a simple yet strongly binding electron donor-acceptor (EDA) complex, BH(3)NH(3) serves as a good example to study the electron pair donor-acceptor complexes. We employed both the ab initio valence bond (VB) and block-localized wave function (BLW) methods to explore the electron transfer from NH(3) to BH(3). Conventionally, EDA complexes have been described by two diabatic states: one neutral state and one ionic charge-transferred state. Ab initio VB self-consistent field (VBSCF) computations generate the energy profiles of the two diabatic states together with the adiabatic (ground) state. Our calculations evidently demonstrated that the electron transfer between NH(3) and BH(3) falls in the abnormal regime where the reorganization energy is less than the exoergicity of the reaction. The nature of the NH(3)-BH(3) interaction is probed by an energy decomposition scheme based on the BLW method. We found that the variation of the charge-transfer energy with the donor-acceptor distance is insensitive to the computation levels and basis sets, but the estimation of the amount of electron transferred heavily depends on the population analysis procedures. The recent resurgence of interest in the nature of the rotation barrier in ethane prompted us to analyze the conformational change of BH(3)NH(3), which is an isoelectronic system with ethane. We found that the preference of the staggered structure over the eclipsed structure of BH(3)NH(3) is dominated by the Pauli exchange repulsion.     

Reactions of diborane with ammonia and ammonia borane: catalytic effects for multiple pathways for hydrogen release

High-level electronic structure calculations have been used to construct portions of the potential energy surfaces related to the reaction of diborane with ammonia and ammonia borane (B2H6 + NH3 and B2H6 + BH3NH3)to probe the molecular mechanism of H2 release. Geometries of stationary points were optimized at the MP2/aug-cc-pVTZ level. Total energies were computed at the coupled-cluster CCSD(T) theory level with the correlation-consistent basis sets. The results show a wide range of reaction pathways for H2 elimination. The initial interaction of B2H6 + NH3 leads to a weak preassociation complex, from which a B-H-B bridge bond is broken giving rise to a more stable H3BHBH2NH3 adduct. This intermediate, which is also formed from BH3NH3 + BH3, is connected with at least six transition states for H2 release with energies 18-93 kal/mol above the separated reactants. The lowest-lying transition state is a six-member cycle, in which BH3exerts a bifunctional catalytic effect accelerating H2 generation within a B-H-H-N framework. Diborane also induces a catalytic effect for H2 elimination from BH3NH3 via a three-step pathway with cyclic transition states. Following conformational changes, the rate-determining transition state for H2 release is approximately 27 kcal/mol above the B2H6 + BH3NH3 reactants, as compared with an energy barrier of approximately 37 kcal/mol for H2 release from BH3NH3. The behavior of two separated BH3 molecules is more complex and involves multiple reaction pathways. Channels from diborane or borane initially converge to a complex comprising the H3BHBH2NH3adduct plus BH3. The interaction of free BH3 with the BH3 moiety of BH3NH3 via a six-member transition state with diborane type of bonding leads to a lower-energy transition state. The corresponding energy barrier is approximately 8 kcal/mol, relative to the reference point H3BHBH2NH3 adduct + BH3. These transition states are 27-36 kcal/mol above BH3NH3 + B2H6, but 1-9 kcal/mol below the separated reactants BH3NH3 + 2 BH3. Upon chemical activation of B2H6 by forming 2 BH3, there should be sufficient internal energy to undergo spontaneous H2 release. Proceeding in the opposite direction, the H2 regeneration of the products of the B2H6 + BH3NH3reaction should be a feasible process under mild thermal conditions.     

Study of the structure, energetics, and vibrational properties of small ammonia clusters (NH3)n (n = 2-5) using correlated ab initio methods

Equilibrium geometries, interaction energies, and harmonic frequencies of (NH3)n isomers (n = 2-5) have been computed using correlated calculations (MP2) in conjunction with Dunning's aug-cc-pVXZ (X = D, T, Q) basis sets and the Counterpoise procedure. Whenever available, literature values for the binding energy and geometry of dimers and trimers agree well with our data. Low lying isomers for (NH3)4 and (NH3)5 have been found to have similar binding energies (roughly 16 and 20 kcal/mol for the tetramer and pentamer, respectively), perhaps suggesting the presence of a very smooth energy landscape. Using BSSE corrected forces or freezing the monomer structure to its gas phase geometry have been found to have only a weak impact on the energetic and structural properties of the clusters. The effect of zero-point energy (ZPE) on the relative stability of the clusters has been estimated using harmonic frequencies. The latter also highlighted the presence of vibrational fingerprints for the presence of double acceptor ammonia molecules. Many-body effects for (NH3)n isomers (n = 2-4) have been investigated to explore the possibility of building a pairwise interaction model for ammonia. In the frame of the work presented, we have found the 3-body effect to account for 10-15% of the total interaction energy, whereas the 4-body effects may be neglected as first approximation.     

Semiclassical extension of the Landau-Teller theory of collisional energy transfer

A semiclassical version of the quantum coupled-states approximation for the vibrational relaxation of diatomic molecules in collisions with monatomic bath gases is presented. It is based on the effective mass approximation and a recovery of the semiclassical Landau exponent from the classical Landau-Teller collision time. For an interaction with small anisotropy, the Landau exponent includes first order corrections with respect to the orientational dependence of the collision time and the effective mass. The relaxation N(2)(v=1)-->N(2)(v=0) in He is discussed as an example. Employing the available vibrationally elastic potential, the semiclassical approach describes the temperature dependence of the rate constant k(10)(T) over seven orders of magnitude across the temperature range of 70-3000 K in agreement with experimental data and quantum coupled-states calculations. For this system, the hierarchy of corrections to the Landau-Teller conventional treatment in the order of importance is the following: quantum effects in the energy release, dynamical contributions of the rotation of N(2) to the vibrational transition, and deviations of the interaction potential from a purely repulsive form. The described treatment provides significant simplifications over complete coupled-states calculations such that applications to more complex situations appear promising.     

Vibrational analyses of sulfamoyl halides NH2SO2X (X is F, Cl and Br)

The structural stability of sulfamoyl halides NH(2)-SO(2)X (X is F, Cl and Br) were investigated by DFT-B3LYP/6-311+G** and ab initio MP2/6-311+G** calculations. From the calculations the molecules were predicted to exist only in the anti (XS bond is anti with respect to nitrogen lone pair) conformation with the possibility of very low abundance of the syn (SO(2) and NH(2) groups eclipse each other) form of only the fluoride. The equilibrium constant for the syn<-->anti conformational conversion of sulfamoyl fluoride was calculated to be 0.0172 that corresponds to an equilibrium mixture of about 2% syn and 98% anti at 298.15K. The vibrational frequencies were computed at DFT-B3LYP level for the stable anti conformer of the d(0) and d(2) (ND(2)-SO(2)X) deuterated species of the three molecules. Normal coordinate calculations were then carried out and the potential energy distributions were calculated for the molecules.     

Infrared photodissociation spectroscopy of Na(NH3)n clusters: probing the solvent coordination

The first mass-selective vibrational spectra have been recorded for Na(NH3)n clusters. Infrared spectra have been obtained for n = 3-8 in the N-H stretching region. The spectroscopic work has been supported by ab initio calculations carried out at both the DFT(B3LYP) and MP2 levels, using a 6-311++G(d,p) basis set. The calculations reveal that the lowest energy isomer for n or= 7 is indicative of molecules entering a second solvation shell, i.e., the inner solvation shell around the sodium atom can accommodate a maximum of six NH3 molecules.     

Scattering resonances in slow NH3-He collisions

We theoretically study slow collisions of NH(3) molecules with He atoms, where we focus in particular on the observation of scattering resonances. We calculate state-to-state integral and differential cross sections for collision energies ranging from 10(-4) cm(-1) to 130 cm(-1), using fully converged quantum close-coupling calculations. To describe the interaction between the NH(3) molecules and the He atoms, we present a four-dimensional potential energy surface, based on an accurate fit of 4180 ab initio points. Prior to collision, we consider the ammonia molecules to be in their antisymmetric umbrella state with angular momentum j = 1 and projection k = 1, which is a suitable state for Stark deceleration. We find pronounced shape and Feshbach resonances, especially for inelastic collisions into the symmetric umbrella state with j = k = 1. We analyze the observed resonant structures in detail by looking at scattering wavefunctions, phase shifts, and lifetimes. Finally, we discuss the prospects for observing the predicted scattering resonances in future crossed molecular beam experiments with a Stark-decelerated NH(3) beam.     

Complexes of molecular and ionic character in the same matrix layer: infrared studies of the sulfuric acid/ammonia system

The atmospherically important interaction products of sulfuric acid and ammonia molecules have been firstly observed by matrix isolation Fourier transform infrared spectroscopy (MIS-FTIR). Infrared spectra of solid argon matrix layers, in which both H(2)SO(4) and NH(3) molecules were entrapped as impurities, were analyzed for bands not seen in matrix layers containing either of the parent molecules alone. Results were interpreted on the basis of spectral changes, experimental conditions, and semiempirically scaled frequencies from the B3LYP/aug-cc-pVTZ and B3LYP/aug-cc-pVQZ calculations. Bands were assigned to complexes of the H(2)SO(4)·NH(3) and H(2)SO(4)·[NH(3)](2) general formulas. They differ significantly: the 1:1 H(2)SO(4)·NH(3) complex is a strongly hydrogen bonded complex, an analogue of the H(2)SO(4)·H(2)O complex, studied previously. For the 1:2 H(2)SO(4)·[NH(3)](2) complex, spectral results indicate an almost complete proton transfer forming a complex of essentially the two ionic moieties HSO(4)(-) and [H(3)N···H···NH(3)](+), an analogue of the [H(2)O···H···OH(2)](+) "Zundel ion".     

Finding important anharmonic terms in the sixth-order potential energy function by the scaled hypersphere search method: an application to vibrational analyses of molecules and clusters

A fitting method of the sixth-order potential energy function is proposed, where ab initio potential energy data for the fitting are sampled in directions containing maximal anharmonic downward distortions detected by the scaled hypersphere search (SHS) method. This technique has been applied to H2O, HCHO, HCOOH, C2H4, CH3OH, CH3CHO, CH3NH2, B2H6, (H2O)2, and (H2O)3, where, without using the symmetry, 176, 904, 1432, 2992, 2520, 2760, 3608, 6232, 768, and 1456 times single-point energy calculations, respectively, were required for obtaining anharmonic terms. Experimental IR peak positions of not only fundamentals but also overtones and combinations in the excitation energy range of 1000-4000 cm(-1) could be reproduced very accurately by the post-vibrational self-consistent field theory employing potential functions obtained by the present SHS based polynomial fitting method.