Ab initioand DFT calculations of some weakly bound dimers and complexes. I. The dimers of ammonia and phosphine

A Tug of War between the Self- and Cross-associating Hydrogen Bonds in Neutral Ammonia-Water Clusters: Energetic Insights by Molecular Tailoring Approach

In the present work, the energies of various types of individual HBs observed in neutral (NH₃ )_m (H₂ O)_n , (m+n=2 to 7) clusters were estimated using the molecular tailoring approach (MTA)-based method. The calculated individual HB energies suggest that the O-H…N HBs are the strongest (1.21 to 12.49 kcal mol^-1 ). The next ones are the O-H…O (3.97 to 9.30 kcal mol^-1 ) HBs. The strengths of N-H…N (1.09 to 5.29 kcal mol^-1 ) and N-H…O (2.85 to 5.56 kcal mol^-1 ) HBs are the weakest. The HB energies in dimers also follow this rank ordering. However, the HB energies in dimers are much smaller than those obtained by the MTA-based method due to the loss in cooperativity contribution in the dimers. Thus, the calculated cooperativity contributions, for different types of HBs, fall in the range 0.64 to 5.73 kcal mol^-1 . We wish to emphasize based on the energetic rank ordering obtained by the MTA-based method that the O-H of water is a better HB donor than the N-H of ammonia. The reasons for the observed energetic rank ordering are two folds: (i) intrinsically stronger O-H…N HBs than the O-H…O ones as revealed by dimer energies and (ii) the higher cooperativity contribution in the former than the later ones. Indeed, the MTA-based method is useful in providing the missing energetic rank ordering of various type of HBs in neutral (NH₃ )_m (H₂ O)_n clusters, in the literature.

A comprehensive theoretical study of positron binding and annihilation properties of hydrogen bonded binary molecular clusters

Small hydrogen inorganic molecules such as water have no positron binding ability. We revealed that their hydrogen bonded binary molecular clusters exhibit greater positron affinities due to the increased dipole moments and polarization effect.

Structures and relative stabilities of ammonia clusters at different temperatures: DFT vs. ab initio

The global minimum energy structures of (NH₃)_n=2–10are pointed out for the first time at a given temperature.

Terahertz Vibration−Rotation−Tunneling Spectroscopy of the Ammonia Dimer. II. A−E States of an Out-of-Plane Vibration and an In-Plane Vibration

Terahertz vibration-rotation-tunneling transitions have been measured between ca. 78.5 and 91.9 cm-1, and assigned to A-E (ortho-para) combinations of NH3 monomer states. The spectrum is complicated by inversion splittings that correlate to E symmetry monomer rovibronic states. Twenty progressions have been assigned to six excited states involving an out-of-plane vibration and an in-plane intermolecular vibration. The quality of the fit was affected by strong Coriolis interactions among these states and possibly an additional K = 2 state that was not explicitly observed in the data.

Raman spectrum of [Ru(CNBu t)(CO)(eta2-C6H4-2-CHO)(PPh3)2][BF4].2CDCl3 shows that the crystallographically determined bifurcated hydrogen-bonding interaction Cl3CD...F2BF2- is an example of a blue-shifting hydrogen bond

Raman data suggest that a crystallographically determined Cl3CD...F2BF2- interaction in the solid-state structure of [Ru(CNBut)(CO)(eta2-C6H4-2-CHO)(PPh3)2][BF4].2CDCl3 is an example of a blue-shifting bifurcated hydrogen bond. The nu(C-D) band blue-shifts 5 cm-1 to 2269 cm-1 compared to 2264 cm-1 for CDCl3 in the gas phase and 20 cm-1 from frozen CDCl3 at 2249 cm-1. A conventional interpretation of these band shifts would suggest that the CCl2 fragment of DCCl3 is a stronger hydrogen-bond acceptor than the BF2 fragment of a BF4- group.

Hydrogen Transfer vs Proton Transfer in 7-Hydroxy-quinoline·(NH3)3: A CASSCF/CASPT2 Study

Multiconfigurational CASSCF and CASPT2 calculations were performed to investigate the enol --> keto tautomerization in the lowest singlet excited state of the 7-hydroxyquinoline.(NH3)3 cluster. Two different reaction mechanisms were explored. The first one corresponds to that proposed previously by Tanner et al. (Science 2003, 302, 1736) on the basis of experimental observations and CASSCF optimizations under Cs-symmetry constraints. This mechanism comprises four consecutive steps and involves nonadiabatic transitions between the valence 1pipi* state and a pisigma* Rydberg-type state, resulting in hydrogen-atom transfer. Single-point CASPT2 calculations corroborate that for Cs-symmetry pathways hydrogen-atom transfer is clearly preferred over proton transfer. The second mechanism, predicted by CASSCF optimizations without constraints, implies proton transfer along a pathway on the 1pipi* surface in which one or more ammonia molecules depart significantly from the molecular plane defined by the hydroxyquinoline ring. The results suggest that both mechanisms may be competitive with proton transfer being somewhat favorable over hydrogen-atom transfer.

Terahertz Vibration−Rotation-Tunneling Spectroscopy of the Ammonia Dimer: Characterization of an out of Plane Vibration

The terahertz vibration-rotation-tunneling (VRT) spectrum of the ammonia dimer (NH(3))(2) has been measured between ca. 78.5 and 91.9 cm(-1). The dipole-allowed transitions are separated into three groups that correspond to the 3-fold internal rotation of the NH(3) subunits. Transitions have been assigned for VRT states of the A-A (ortho-ortho) combinations of NH(3) monomer states. The spectrum is further complicated by strong Coriolis interactions. K = 0 <-- 0, K = 1 <-- 0, K = 0 <-- 1, and K = 1 <-- 1 progressions have been assigned. The band origins, rotational constants, asymmetry doubling, centrifugal distortion, and Coriolis coupling constant have been determined from the fit to an effective Hamiltonian. These VRT transitions are tentatively assigned to an out of plane vibration with a K = 0 state at 89.141305(47) cm(-1), and a K = 1 state at 86.77785(9) cm(-1).

Complex Formation of Trimethylaluminum and Trimethylgallium with Ammonia: Evidence for a Hydrogen-Bonded Adduct

We have investigated the formation of gas-phase adducts of trimethylaluminum and trimethylgallium with ammonia using room-temperature Fourier transform infrared experiments and density functional theory calculations. Our results indicate for the first time that, at higher partial pressures, a product distinct from the well-known (CH3)3M:NH3 adduct grows in for both M = Al and M = Ga. Comparison of the experimental and calculated IR spectra, along with calculations of the energetics, indicates that this second product is the result of hydrogen bonding of a second NH3 molecule to the (CH3)3M:NH3 adduct and can be written as (CH3)3M:NH3...NH3. The binding energy of this hydrogen-bonded adduct is calculated to be 26.8 kcal/mol for M = Al and 18.4 kcal/mol for M = Ga and is lower in energy (more stable) relative to the 1:1 (CH3)3M:NH3 adduct by 7.2 kcal/mol for M = Al and 6.6 kcal/mol for M = Ga. In contrast, an alternative complex involving the formation of two separate M-N donor-acceptor bonds, which is written as H3N:(CH3)3M:NH3, is calculated to be lower in energy relative to (CH3)3M:NH3 by only 0.1 kcal/mol for M = Al and 0.2 kcal/mol for M = Ga and is not observed experimentally. These results show that hydrogen bonding plays an important role in the interaction of ammonia with metal organic precursors involving Al, Ga, and In, under typical metal organic chemical vapor deposition AlGaInN growth conditions.