L. Massa

Dielectric Response of High Explosives at THz Frequencies Calculated Using Density Functional Theory

We present in this study calculations of the ground-state resonance structures associated with the high explosives β-HMX, PETN, RDX, TNT1, and TNT2 using density functional theory (DFT). Our objective is the construction of parameterized dielectric-response functions for excitation by electromagnetic waves at compatible frequencies. These dielectric-response functions provide the basis for analyses pertaining to the dielectric properties of explosives. In particular, these dielectric-response functions provide quantitative initial estimates of spectral-response features for subsequent adjustment with knowledge of additional information, such as laboratory measurements and other types of theory-based calculations. With respect to qualitative analyses, these spectra provide for the molecular-level interpretation of response structure. The DFT software GAUSSIAN was used for the calculations of the ground-state resonance structures presented here.

syn−anti Conversion in Octahedral Bis(β-diketonato)diorganotin(IV) Derivatives Containing Fluorinated 4-Acyl-5-pyrazolonato Donors

Six-coordinate organotin derivatives L2SnR2 [L = 3-methyl-1-(4-trifluoromethylphenyl)-4-R3-C=O-5-pyrazolonato (R3 = CH3, L = L1; R3 = C6H5, L = L2; R3 = CF3, L = L3; R = CH3, n-C4H9, C6H5)] have been synthesized and characterized by analytical and spectroscopic (1H, 13C, 119Sn, and 19F NMR, IR) techniques. Partial dissociation of one ligand in chloro-hydrocarbon solvents gives rise to cationic five-coordinate L-(solvent)-diorganotin(IV) complexes. The X-ray crystal structure of bis[4-benzoyl-3-methyl-1-(4-trifluoromethylphenyl)pyrazolon-5-ato]diphenyltin (6) shows the metal in a distorted octahedron (skewed trapezoidal bipyramidal) with the two β-diketonato donors in syn positions, a C−Sn−C bond angle of 165.2(2)° and two sets of tin−oxygen bonds [2.141(5) and 2.139(3) A for Sn−O(pyrazolonato) and 2.250(4) and 2.272(5) A for Sn−O(acyl)]. Surprisingly, the recently reported dimethyltin derivative has the anti configuration, in contrast to expectations based on all previous experience. Large scale Hartree−Fock (HF) and Density Functional Theory (DFT) calculated structures for the anti configuration of bis[4-benzoyl3-methyl-1-(4-trifluoromethylphenyl)pyrazolon-5-ato]dimethyltin show good agreement with the experimental structure obtained from X-ray methods. The hypothetical syn configuration of bis[4-benzoyl-3-methyl-1-(4-trifluoromethylphenyl)pyrazolon-5-ato]dimethyltin was also studied theoretically and both methods also predict characteristic structural features, such as: (from HF) a C−Sn−C bond angle of 150.0°, Sn−O(pyrazolonato) bond lengths of 2.088 and 2.084 A and Sn−O(acyl) bond lengths of 2.385 and 2.401 A. Results from the syn calculated structures suggest that intramolecular repulsive F···F interactions contribute to the syn−anti conversion.

A Novel Configuration of a Benzoylacetonato-Diorganotin Species is Modified by an Electron-Withdrawing Substituent on Tin − Synthesis, IR and NMR Spectroscopy, Structure, and ab initio Studies

Diorganotin(IV) complexes of the β-diketonato benzoylacetonato ligand were synthesized and characterized with IR and multinuclear (H, C, Sn) NMR spectroscopy. The X-ray diffraction study of bis(benzoylacetonato)di-tert-butyltin(IV) shows two independent molecules in the crystallographic unit cell. The metal polyhedron is a distorted octahedral (Skewed Trapezoidal Bipyramidal) with the trans angle C−Sn−C of 151.5(5)°. Each ligand chelates the metal with different donor abilities [Sn−O bond lengths of 2.423(8) A and 2.135(8) A in one ligand, and 2.107(9) A and 2.357(8) A in the other]; results for the 2nd molecule are similar. The coordination arrangement differs from those of related bis(β-diketonato)diorganotin derivatives containing asymmetric ligands, which are characterized by an approximate Cs symmetry, in that both benzoylacetonato ligands point their methyl (and phenyl) substituents across (anti) the metal atom. Chelate planarity and some phenyl-chelate co-planarity was observed. Hartree−Fock (HF) and Density Functional Theory (DFT) calculated structures are in good agreement with those obtained using X ray analysis. DFT and HF calculated structures of bis(benzoylacetonato)diphenyltin(IV), having a similar connectivity as bis(benzoylacetonato)di-tert-butyltin(IV), differ markedly: the phenyls subtend a C−Sn−C bond angle of 180°, all Sn−O bond lengths are equal and the tin coordination sphere is a regular octahedron with the ligand bzac being isobidentate. This arrangement is caused by the phenyl electronic withdrawal effect. However, IR data for this compound show several Sn−O bands inconsistent with this configuration and suggest a different connectivity. Additional DFT structural calculations for (bzac)2SnPh2, performed on the connectivity resembling the “normal” syn geometry, show a conformational energy slightly lower than that of the centrosymmetric arrangement. Low temperature NMR spectra illustrate the possible rearrangements in solution. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Issues concerning spectral analysis of water samples for monitoring and treatment of public water resources

Experimental measurements conducted in the laboratory, involving hyperspectral analysis of water samples taken from public water resources, have motivated a re-evaluation of issues concerning the potential application of this type of analysis for water monitoring, treatment and evaluation prior to filtration. One issue concerns hyperspectral monitoring of contaminants with respect to types and relative concentrations. This implies a need to better understand the statistical profiles of water contaminants in terms of spatial-temporal distributions of electromagnetic absorption spectra ranging from the ultraviolet to infrared, which are associated with specific water resources. This issue also implies the need to establish correlations between hyperspectral signatures and types of contaminants to be found within specific water resources. Another issue concerns the use of absorption spectra to determine changes in chemical and physical characteristics of contaminants after application of water treatments, in order to determine levels of toxicity with respect to the environment. This paper presents a prototype spectral analysis showing various aspects relevant to water monitoring and discusses the use of basic theory for the interpretation of spectral features associated with water contaminants, as well as discussing inverse analysis of hyperspectral measurements.

Form factors for core electrons useful for the application of quantum crystallography (QCr) to organic molecules.

Form factors are calculated for the core electrons of the first-row atoms B, C, N, O and F. The form factors are presented in an analytical form, as appears in International Tables for X-ray Crystallography [Ibers & Hamilton (1974), Vol. IV, pp. 103-145. Birmingham: Kynoch Press; present distributor Kluwer Academic Publishers, Dordrecht]. Having such form factors for the core electrons reduces the number of parameters to be determined by the methods of quantum crystallography (QCr). It is shown that the form factors obtained are quite accurate. Thus, when they are used in QCr, they are expected to be consistent with accurate charge densities.

Quantum Crystallography: Features and Application

Quantum crystallography (QCr) is an area of research that arises from the fact that experimental X-ray diffraction data obtained from crystals can also be readily described theoretically by the use of quantum mechanical modeling. The intimate connection between experiment and theory arises from the fact that X-rays are scattered by electrons whose distributions are represented in the experimental data and models of electron density distributions are given by quantum mechanics (Q.M.). An objective of this type of research is to obtain a quantum mechanical model that is consistent with the crystallographic data, thus affording the opportunity to calculate numerous properties of interest, for example, various energies, electron distributions, atomic charges and electrostatic potentials. Our approach to quantum crystallography is based on the use of a single, idempotent density matrix (a projector matrix) [1]. In the initial stages of the process of optimizing the fit of a quantum mechanical model to X-ray diffraction data, it is valuable to have a projector matrix that is as close as possible to the one that results from the fitting process. Such a matrix is obtainable from ab initio calculations. The fitting process involves the adjustment of the values of the elements in the projector matrix and certain other parameters while preserving the idempotency of the matrix and its normalized trace. These properties will be described later on.

IR absorption spectra for SixOy-nH2O molecular clusters using density functional theory

Calculations are presented of vibrational absorption spectra for energy minimized structures of SixOy-nH2O molecular clusters using density function theory (DFT). DFT can provide interpretation of absorption spectra with respect to molecular structure for excitation by electromagnetic waves at frequencies within the IR range. The absorption spectrum corresponding to excitation states of SixOy-nH2O molecular clusters consisting of relatively small numbers of atoms should be associated with response features that are intermediate between that of isolated molecules and that of a bulk system. DFT calculated absorption spectra represent quantitative estimates that can be correlated with additional information obtained from laboratory measurements. The DFT software GAUSSIAN was used for the calculations of excitation states presented here.

Calculation of Vibrational and Electronic Excited-State Absorption Spectra of Arsenic-Water Complexes Using Density Functional Theory

Calculations are presented of vibrational and electronic excited-state absorption spectra for As-H2O complexes using density function theory (DFT) and time-dependent density functional theory (TD-DFT). DFT and TD-DFT can provide interpretation of absorption spectra with respect to molecular structure for excitation by electromagnetic waves at frequencies within the IR and UV-visible ranges. The absorption spectrum corresponding to excitation states of As-H2O complexes consisting of relatively small numbers of water molecules should be associated with response features that are intermediate between that of isolated molecules and that of a bulk system. DFT and TD-DFT calculated absorption spectra represent quantitative estimates that can be correlated with additional information obtained from laboratory measurements and other types of theory based calculations. The DFT software GAUSSIAN was used for the calculations of excitation states presented here.

Calculation of electronic-excited-state absorption spectra of water clusters using time-dependent density functional theory

Calculations are presented of electronic-excited-state absorption spectra for molecular clusters of H2O using time-dependent density functional theory (TD-DFT). Calculation of excited state resonance structure using TD-DFT can provide interpretation of absorption spectra with respect to molecular structure for excitation by electromagnetic waves at frequencies within the UV-visible range. The absorption spectrum corresponding to electronic excitation states of a molecular cluster consisting of a relatively small number of water molecules should be associated with response features that are intermediate between that of isolated molecules and that of a bulk lattice. TD-DFT calculated absorption spectra represent quantitative estimates that can be correlated with additional information obtained from laboratory measurements and other types of theory based calculations. The DFT software GAUSSIAN was used for the calculations of electronic excitation states presented here.

Prototype spectral analysis of water samples for monitoring and treatment of public water resources

Experimental measurements conducted in the laboratory, involving hyperspectral analysis of water samples taken from public water resources in the New York City metro area, have motivated a reevaluation of issues concerning the potential application of this type of analysis for water monitoring, treatment and evaluation prior to filtration. One issue concerns hyperspectral monitoring of contaminants with respect to types and relative concentrations. This implies a need for better understanding the statistical profiles of water contaminants in terms of spatial-temporal distributions of electromagnetic absorption spectra ranging from the ultraviolet to infrared, which are associated with specific water resources. This issue also implies the need for establishing correlations between hyperspectral signatures and types of contaminants to be found within specific water resources. Another issue concerns the use of absorption spectra for determining changes in chemical and physical characteristics of contaminants after application of water treatments in order to determine levels of toxicity with respect to the environment.