Nguyen-Nguyen Pham-Tran

PCCP and its isomers: a theoretical studyElectronic Supplementary Information available. See http://www.rsc.org/suppdata/cp/b1/b106927f/

Structural and electronic features of the linear 1,4-diphosphabutadiyne PC–CP and its rhombic isomer were explored using ab initio quantum chemical methods. Portions of the lower-lying [C2P2] potential energy surfaces of both neutral and ionized states were constructed. Geometries, rotational constants, vibrational frequencies, proton affinities and heats of formation of the ground state planar structures were determined using B3LYP and CCSD(T) computations. The vertical excitation and ionization energies were evaluated using the equation of motion EOM-CCSD. Atomic basis sets of different qualities ranging from 6-311++G(d,p) to aug-cc-pV6Z, were employed. Some thermochemical parameters were evaluated as follows: heat of formation at 0 K ΔfH0(PCCP) = 413 ± 8 kJ/mol−1, adiabatic ionization energy IEa(PCCP) = 9.6 ± 0.2 eV, proton affinity PA(PCCP) = 715 ± 8 kJ mol−1. The heats of formation at 0 K of relevant species are: ΔfH0(CP) = 510 kJ mol−1 and ΔfH0(HCP) = 225 kJ mol−1, and the dissociation energies, De(H–CP) = 501 kJ mol−1 and De(PC–CP) = 606 kJ mol−1, with expected errors of ±10 kJ mol−1.

Ab Initio Investigation of O-H Dissociation from the Al-OH2 Complex Using Molecular Dynamics and Neural Network Fitting.

The dissociation dynamics of the O-H bond in Al-OH2 is investigated on an approximated ab initio potential energy surface (PES). By adopting a dynamic sampling method, we obtain a database of 92 834 configurations. The potential energy for each point is calculated using MP2/6-311G (3df, 2p) calculations; then, a 60-neuron feed-forward neural network is utilized to fit the data to construct an analytic PES. The root-mean-square error (rmse) for the training set is reported as 0.0036 eV, while the rmse for the independent testing set is 0.0034 eV. Such excellent fitting accuracy indeed confirms the reliability of the constructed PES. Subsequently, quasi-classical molecular dynamics (MD) trajectories are performed on the constructed PES at various levels of vibrational excitation in the range of 1.03 to 2.23 eV to investigate the probability of O-H bond dissociation. The results indicate a linear relationship between reaction probability and internal energy, from which we can determine the minimum activation internal energy required for the dissociation as 0.62 eV. Moreover, the O-H bond rupture is shown to be highly correlated with the formation of Al-O bond.

Theoretical study of cyanophosphapropyne (NCCP), isocyanophosphapropyne (CNCP) and their isomers: stability and properties

Exploration of portions of the (C2NP) potential energy surface using both B3LYP and CCSD(T) methods with the 6-311++G(d,p) basis set, indicates that cyanophosphapropyne NC–CP is the most stable isomer, followed by isocyanophosphapropyne CN–CP, the linear azaphosphadicarbon CCNP and the bent isocyanophosphavinylidene NC–PC. These higher-lying isomers are relatively stable with respect to unimolecular rearrangements and fragmentations. Their molecular properties including the geometries, rotational constants, vibrational wavenumbers, 13C and 31P NMR chemical shifts, heats of formation, excitation and ionisation energies, proton and electron affinities were determined. For the thermochemical quantities, CCSD(T) and EOM-CCSD computations with larger 6-311++G(3df,2p) and aug-cc-pVTZ basis sets were employed. It is remarkable that as a substituent, the phosphaethynyl –CP moiety exerts a remarkably strong electron donor effect which markedly enhances the electron density and basicity of the attaching moieties. Thus, the proton affinities at N in NC–CP and at C in CN–CP are significantly increased thanks to the CP effect whereas those at C of CP are, as a consequence, strongly reduced.

Theoretical and Experimental Study of the Conformation and Vibrational Frequencies of N‐Acetyl‐L‐alanine and N‐Acetyl‐L‐alaninate

The vibrational frequencies of N‐acetyl‐L‐alanine (NAAL), its potassium salt (NAALK) and its free anionic form (NAAL−) are calculated using density functional theory (B3LYP) combined with the 6‐311 + + G(d,p) basis set. The experimental Raman spectrum of solid NAALK and the scaling factors for calculated values are discussed as well. The three species are characterized by intramolecular NH…O hydrogen bonds leading to the formation of a five‐membered ring. As indicated by the intramolecular (N)H…O distances and by the ν(NH) frequencies, the strength of the intramolecular hydrogen bond is ordered as follows: NAAL− < NAALK < NAAL−. Owing to their difference in the coupling with other vibrational modes, the in‐plane and out‐of‐plane vibrations do not reflect the strength of the hydrogen bond.