Sergio A. González

Secondary Hydrogen Isotope Effects on the Structure and Stability of Cation-π Complexes (Cation = Li+, Na+, K+ and π = Acetylene, Ethylene, Benzene)

Secondary hydrogen isotope effects on the geometries, electronic wave functions and binding energies of cation-pi complexes (cation = Li(+), Na(+), K(+) and pi = acetylene, ethylene, benzene) are investigated with NEO/HF and NEO/MP2 methods. These methods determine both electronic and nuclear wave functions simultaneously. Our results show that an increase of the hydrogen nuclear mass leads to the elongation of the cation-pi bond distance and the decrease in its binding energy. An explanation to this behavior is given in terms of the changes in the pi-molecule electronic structure and electrostatic potential induced by isotopic substitutions.

Turning symmetric an asymmetric hydrogen bond with the inclusion of nuclear quantum effects: The case of the [CN···H···NC]− complex

Nuclear quantum effects (NQE) on the geometry, energy, and electronic structure of the [CN·L·NC](-) complex (L = H, D, T) are investigated with the recently developed APMO/MP2 code. This code implements the nuclear molecular orbital approach (NMO) at the Hartree-Fock (HF) and MP2 levels of theory for electrons and quantum nuclei. In a first study, we examined the H/D/T isotope effects on the geometry and electronic structure of the CNH molecule at NMO/HF and NMO/MP2 levels of theory. We found that when increasing the hydrogen nuclear mass there is a reduction of the R(N-H) bond distance and an increase of the electronic population on the hydrogen atom. Our calculated bond distances are in good agreement with experimental and other theoretical results. In a second investigation, we explored the hydrogen NQE on the geometry of [CNHNC](-) complex at the NMO/HF and NMO/MP2 levels of theory. We discovered that while a NMO/HF calculation presented an asymmetric hydrogen bond, the NMO/MP2 calculation revealed a symmetric H-bond. We also examined the H/D/T isotope effects on the geometry and stabilization energy of the [CNHNC](-) complex. We noted that gradual increases in hydrogen mass led to reductions of the R(NN) distance and destabilization of the hydrogen bond (H-bond). A discussion of these results is given in terms of the hydrogen nuclear delocalization effects on the electronic structure and energy components. To the best of our knowledge, this is the first ab initio NMO study that reveals the importance of including nuclear quantum effects in conventional electronic structure calculations for an enhanced description of strong-low-barrier H-bonded systems.

First principles investigation of hydrogen isotope effects in [XSO4–H–SO4X]− (X = H, K) complexes

Hydrogen isotope effects on geometries, total energies, nuclear and electronic wave functions of the [HO3SO–H–OSO3H]− and [KO3SO–H–OSO3K]− complexes are investigated with the NEO/HF method. This method determines both electronic and nuclear wave function simultaneously. A discussion of the isotope effects is provided and used to explain the hydrogen isotope effects on the phase transition temperatures in hydrogen bonded ferroelectric materials, K3H(SO4)2 and K3D(SO4)2.

Atmospheric chemistry of CF3CF2CHO: absorption cross sections in the UV and IR regions, photolysis at 308 nm, and gas-phase reaction with OH radicals (T = 263-358 K).

The relative importance in the atmosphere of UV photolysis of perfluoropropionaldehyde, CF3CF2CHO, and reaction with hydroxyl (OH) radicals has been investigated in this work. First, the forbidden n → π* transition of the carbonyl chromophore was characterized between 230 and 380 nm as a function of temperature (269–298 K) and UV absorption cross sections, σλ, were determined in those ranges. In addition, IR absorption cross sections were determined between 4000 and 500 cm–1. Pulsed laser photolysis (PLP) of CF3CF2CHO coupled to Fourier transform infrared (FTIR) was employed to determine the overall photolysis quantum yield, Φλ, at 308 nm and 298 K. Φλ=308 nm was pressure dependent, ranging from (0.94 ± 0.14) at 75 Torr to (0.30 ± 0.01) at 760 Torr. This dependence is characterized by the Stern–Volmer parameters Φ0λ=308 nm = (1.19 ± 0.34) and KSV = (1.22 ± 0.52) × 10(–19) cm3 molecule–1. End products of the photodissociation of CF3CF2CHO were measured and quantified by FTIR spectroscopy. Furthermore, the rate coefficients for the OH + CF3CF2CHO reaction, k1, were determined as a function of temperature (T = 263–358 K) by PLP-LIF. At room temperature the rate coefficient is k1(T = 298 K) = (5.57 ± 0.07) × 10(–13) cm3 molecule–1 s–1, whereas the temperature dependence is described by k1(T) = (2.56 ± 0.32) × 10(–12) exp{−(458 ± 36)/T} cm3 molecule–1 s–1. On the basis of our results, photolysis of CF3CF2CHO in the actinic region could be an important removal process for CF3CF2CHO in the atmosphere. The formation of the primary products in the UV photolysis of CF3CF2CHO is also discussed.

Atmospheric Degradation Initiated by OH Radicals of the Potential Foam Expansion Agent, CF3(CF2)2CH═CH2 (HFC-1447fz): Kinetics and Formation of Gaseous Products and Secondary Organic Aerosols

The assessment of the atmospheric impact of the potential foam expansion agent, CF3(CF2)2CH═CH2 (HFC-1447fz), requires the knowledge of its degradation routes, oxidation products, and radiative properties. In this paper, the gas-phase reactivity of HFC-1447fz with OH radicals is presented as a function of temperature, obtaining kOH (T = 263-358 K) = (7.4 ± 0.4) × 10(-13)exp{(161 ± 16)/T} (cm(3)·molecule(-1)·s(-1)) (uncertainties: ±2σ). The formation of gaseous oxidation products and secondary organic aerosols (SOAs) from the OH + HFC-1447fz reaction was investigated in the presence of NOx at 298 K. CF3(CF2)2CHO was observed at low- and high-NOx conditions. Evidence of SOA formation (ultrafine particles in the range 10-100 nm) is reported with yields ranging from 0.12 to 1.79%. In addition, the absolute UV (190-368 nm) and IR (500-4000 cm(-1)) absorption cross-sections of HFC-1447fz were determined at room temperature. No appreciable absorption in the solar actinic region (λ > 290 nm) was observed, leaving the removal by OH radicals as the main atmospheric loss process for HFC-1447fz. The major contribution of the atmospheric loss of HFC-1447fz is due to OH reaction (84%), followed by ozone (10%) and chlorine atoms (6%). Correction of the instantaneous radiative efficiency (0.36 W m(-2)·ppbv(-1)) with the relatively short lifetime of HFC-1447fz (ca. 8 days) implies that its global warming potential at a time horizon of 100 year is negligible (0.19) compared to that of HCFC-141b (782) and to that of modern foam-expansion blowing agents (148, 882, and 804 for HFC-152a, HFC-245fa and HFC-365mfc, respectively).