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Molecular structures and excited states of CpM(CO)(2) (Cp = eta(5)-C(5)H(5); M = Rh, Ir) and [Cl(2)Rh(CO)(2)](-) complexes have been investigated using the B3LYP and the symmetry-adapted cluster (SAC)/SAC-configuration interaction (SAC-CI) theoretical methods. All the dicarbonyl complexes have singlet ground electronic states with large singlet-triplet separations. Thermal dissociations of CO from the parent dicarbonyls are energetically unfavorable. CO thermal dissociation is an activation process for [Cl(2)Rh(CO)(2)](-) while it is a repulsive potential for CpM(CO)(2). The natures of the main excited states of CpM(CO)(2) and [Cl(2)Rh(CO)(2)](-) are found to be quite different. For [Cl(2)Rh(CO)(2)](-), all the strong transitions are identified to be metal to ligand CO charge transfer (MLCT) excitations. A significant feature of the excited states of CpM(CO)(2) is that both MLCT excitation and a ligand Cp to metal and CO charge transfer excitation are strongly mixed in the higher energy states with the latter having the largest oscillator strength. A competitive charge transfer excited state has therefore been identified theoretically for CpRh(CO)(2) and CpIr(CO)(2). The wavelength dependence of the quantum efficiencies for the photoreactions of CpM(CO)(2) reported by Lees et al. can be explained by the existence of two different types of excited states. The origin of the low quantum efficiencies for the C-H/S-H bond activations of CpM(CO)(2) can be attributed to the smaller proportion of the MLCT excitation in the higher energy states.

Density functional theory methods are employed to investigate experimentally proposed mechanisms by which the antitumor drug tirapazamine may react with a DNA sugar-C(1)' radical to give the sugar derivative deoxyribonolactone, with concomitant DNA strand breakage. For the previously proposed minor pathway, ionization of the sugar-C(1)' radical by tirapazamine, the calculated ionization energy, and the electron affinity of the models of the sugar-C(1)' radical of DNA and tirapazamine suggest that tirapazamine must be protonated to be able to oxidize the sugar-C(1)' radical. The preferred mechanism for reaction of tirapazamine with a sugar-C(1)' radical, in agreement with experimental observations, is found to proceed by direct attack of an N-oxide oxygen of tirapazamine at the sugar-C(1)' position, followed by homolytic cleavage of the N-O bond of the drug moiety. Possible alternative mechanisms are also investigated.

After nearly four decades of curiosity-driven research, electron-pair densities are about to become part of the incredible wealth of algorithms and computer codes that constitute contemporary electronic structure methods. The earliest studies of electron-pair densities focused on the relative motion of two electrons and aimed at gaining insights into the physical consequences of electron correlation. More recently, the balance has shifted in the direction of using information from electron-pair densities to develop faster computational methods for high-level, many-electron calculations.

The interaction of BeX(2) (X = H, F) with water molecules has been analyzed at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. The formation of strong beryllium bonds between water molecules and the BeX(2) derivative triggers significant electron density redistribution within the whole system, resulting in significant changes in the proton donor and proton acceptor capacity of the water molecules involved. Hence, significant cooperative and anti-cooperative effects are present, explaining why there is no case in which the global minimum corresponds to a tetracoordinated beryllium atom. In fact, the most stable clusters can be viewed as the result of the attachment of BeX(2) to the water trimer and the water dimer, respectively, and not as the result of the solvation of the BeX(2) molecule. We have also shown that the decomposition of the interaction energy into atomic components is a reliable quantitative tool to describe all the closed-shell interactions present in the clusters investigated herein, namely hydrogen bonds, beryllium bonds and dihydrogen bonds. Indeed, we have shown that the changes in the atomic energy components are correlated with the changes in the strength of these interactions, and they provide a quantitative measure of cooperative effects directly in terms of energies.

In the above comment Close et al. raise criticism against the comparisons made between theoretical results of various radical forms of cytosine1 with experimental EPR and ENDOR data primarily obtained in single crystals of cytosine monohydrate.2 The essence of the comments made are certainly sound, namely, that there are differences between some of the observations made. Primarily, the authors claim that OH radical adducts are unlikely to be formed in the monohydrate crystals, and that the net effect of ionization is the removal of one hydrogen from the N1 position of some cytosines, and the addition of the H radicals to the N3 position of other cytosines. The formation of N1 dehydrogenated radicals was questioned in the theoretical study1 to which the comment was made. The main source for the difference between the experimentally and theoretically deduced hyperfine coupling constants (HFCC) is, according to Close et al., that the amino groups of cytosine (N4) should be planar from interactions within the crystals, whereas calculations in vacuo render slightly puckered structures. Employing planar models for the N1 dehydrogenated and N3 hydrogenated radicals is claimed to render theoretical data that will support the interpretations made by Close et al. We present new data below that partly supports and partly disagrees with these statements. For the N3 hydrogenated species (labeled N3H), we can see from Table 1 that the large isotropic coupling arising from the out-of-plane amino proton vanishes. The remaining proton couplings undergo minor adjustments from the planarity, as expected, and the overall agreement with ENDOR data is most satisfactory. We may thus conclude, even more strongly than previously in ref 1, that the N3H radical is one of the major species formed when exposing crystalline cytosine monohydrate to ionizing radiation. Performing the same type of computation for the N1 dehydrogenated species (N1), Table 2, does however not lead to any improvements over previous results. The rational for this is that the puckering is very small already for the unconstrained geometry, and the net effect is thus minor. The out-of-plane angle H-N4-C4-N3 (for atomic labeling, see ref 1) of unconstrained N1 is 8.5°. In neutron diffraction experiments on crystalline cytosine monohydrate,3 the HNCN out-of-plane angle was determined to be 3.4°, and a slight puckering of the ring was observed. Including hydrogen bonding moieties in the optimization calculations (four water molecules surrounding the N1 radical of cytosine) lowered the amino group puckering to HNCN ) 3.2 angles. As expected, no major change in HFCCs was observed. From this data, together with the results listed in Table 2, we may thus conclude that planarity itself does not TABLE 1. Comparison of Calculated HFCCs (gauss) for the N3-Hydrogenated Radical (N3H) in Puckered and a Planar Geometries with Experimental Results (Reference 2)

The local quantum theory is applied to the study of the momentum operator in atomic systems. Consequently, a quantum-based local momentum expression in terms of the single-electron density is determined. The limiting values of this function correctly obey two fundamental theorems: Kato's cusp condition and the Hoffmann-Ostenhof and Hoffmann-Ostenhof exponential decay. The local momentum also depicts the electron shell structure in atoms as given by its local maxima and inflection points. The integration of the electron density in a shell gives electron populations that are in agreement with the ones expected from the Periodic Table of the elements. The shell structure obtained is in agreement with the higher level of theory computations, which include the Kohn-Sham kinetic energy density. The average of the local kinetic energy associated with the local momentum is the Weizsacker kinetic energy. In conclusion, the local representation of the momentum operator provides relevant information about the electronic properties of the atom at any distance from the nucleus.

The bonding interaction between non-hydrogen-bonded (NHB) hydrogen atoms and the non-nuclear attractor (NNA) observed in cavity-bound anionic water clusters has been investigated using ab initio methods. The clusters range in size from four to ten water molecules and generally consist of several water molecules hydrogen-bonded in a ring, with two of these rings stacked such that the NHB hydrogen atoms are directed towards a central cavity. Analysis of the electron density using the theory of atoms in molecules yields information about the bonding interaction that cannot be obtained with conventional computational techniques. The results indicate that this novel interaction, Ha...NNA, is a weak, closed-shell interaction. Further analysis of the atomic properties of the NHB hydrogen atoms indicate that this novel bond cannot be classified as a hydrogen bond. On the basis of the results from the current investigation, this interaction is best described as a weak dipole-ion interaction between the positively charged NHB hydrogen atoms and the negatively charged NNA.

Extensive abinitio molecular orbital calculations on six sulfonyl radicals (XSO2 where X = H, CH3, NH2, OH, F, and Cl), the simplest sulfinic acid HSO2H3 and its isomeric sulfone H2SO2, the HSO2− anion of sulfinic acid, the isomeric anion SO2H−, and for completeness, the SO2H radical are presented. By use of the STO-3G* basis set, which includes d-type polarization functions on second-row atoms, all geometrical parameters are varied until the total energy is minimized, subject only to certain symmetry restrictions specified for each system. The inclusion of d orbitals on S is observed to affect the S—O bond lengths by as much as 0.45 Å. The calculations suggest that the radical site in sulfonyl radicals is significantly delocalized over the entire functional group and that the geometrical parameters of the SO2 functional group in sulfonyl radicals are nearly independent of the substituent (r(S—O) = 1.47 ± 0.01 Å, 3SO2, NH2SO2, and OHSO2 are consistent with the chemistry of alkylsulfonyl and aminosulfonyl radicals and lead to an interesting prediction for alkoxylsulfonyl radicals. Furthermore the calculations yield lower total energies for HSO2H, HSO2−, and SO2H than for the respective isomeric forms H2SO2, SO2H−, and HSO2.

The STO-3G optimized structures of three series of carbon acids, CHnX4−n, where n = 1, 2, or 3 and X = F, CN, or NO2 and their corresponding conjugate bases are compared with the limited number of experimentally determined structures. The 6-31G* equilibrium geometries of [Formula: see text] are included as a check on the reliability of the STO-3G structures of the anions. Although the effects of successive substitutions on the STO-3G structures are generally systematic, a few apparent anomalies are readily explained in terms of resonance stabilization. Calculations at the 3-21G level, on the STO-3G optimized structures, indicate that the stabilization energy associated with increasing the number of orbitals in the basis set is approximately additive. The stabilization energies are 0.865 ± 0.007 au per fluorine substituent, 0.661 ± 0.008 au per cyano substituent, and 1.561 ± 0.027 au per nitro substituent.

The results of a detailed analysis of typical situations in crystals containing trigonal-pyramidal YEL3ε species are used to argue that, perse, the lone electron pair (E, LP) on the Y atom determines neither the crystal structure nor the orientation of the YEL3ε units in the crystal. Considered as a space-filling entity, the LP has practically no effective volume of its own. Whatever space-filling or orienting properties may be associated with the presence of the LP, they are not autonomous; they arise from the intrinsic stereochemical involvement of the LP in the geometry of and the charge distribution in the free YEL3ε ion or molecule and hence from the consequences of this geometry for crystal packing. Abinitio (6-31G*) calculations show that the electron density in the region of the LP in an ion is only slightly perturbed by the charge on the counter ion at distances observed in crystals. There is considerable similarity in the electron density distribution in the LP region of Y between free anionic (SO32−) and free cationic (Me3S+) species. The problem of the so-called stereochemical inertness of the LP in the 14-electron octahedral YEX62− ions in crystals is examined in detail. It is concluded that the crystallographic Oh symmetry of the ion in cubic crystals is the result of dynamic averaging of possible orientations of the inherently noncubic YX62− octahedron (cf. the abinitio results for SeCl62−). Such averaging also appears to be responsible for the stereochemical "activation" or "inactivation" of the LP in thermally induced crystallographic transitions in the AYX3 halide perovskites and related crystals. To provide a firmer basis for the discussion of the LP, the crystal structures of the five title compounds have been determined. The crystal chemistry of these and related compounds is discussed. Keywords: abinitio calculations, Me3S+ and SeCl62−; group V trihalides; lone electron pair; trialkylsulfonium salts; VSEPR theory.

Analysis of the partial Fermi hole in the 23S state of helium and the ground states of Li and Be shows that the radius of the Fermi hole increases with the distance of the reference electron from the nucleus, and that there exists a non-zero lower limit for the radius. The limit increases in a regular manner for the two-, and three- and four-electron isoelectronic sequences. The radius of the Fermi hole in the ground states of the Li and Be isoelectronic sequences varies with the atomic number but is independent of the number of electrons. The latter observations stands in contrast to the differences in the sizes of the electron clouds of each isonuclear pair.

A new definition of the Fermi hole in many-electron systems is provided in terms of the distribution function falpha alpha (r12) of the interelectronic distance for electrons with parallel spins. By analogy with the Coulomb hole, the Fermi hole is defined as the difference between the values of f22(r12) derived from the Hartree-Fock and the Hartree wavefunctions. this definition, unlike previous ones, provides a simple picture of the Fermi hole as a function of r12. By assuming that the Hartree and Hartree-Fock orbitals are identical, an analytical formula is derived for the Fermi hole. Explicit calculations are presented for the 23S state of He and the ground state of Be. It is observed that the Fermi hole is remarkably similar in these two cases; and that the effects of Coulomb correlation are more long-ranged than those of Fermi correlation.

The pair distribution function h(r12;r1, γ) and the virial theorem are used to derive a general expression for the local contributions to the total correlation energy of an atom. A direct link between correlation effects and the correlation energy is obtained by use of G(r1, γ) and Γ(r1, y). The former is the probability associated with a given choice of r1 and γ, while the latter describes the local contribution to the correlation energy. Explicit calculations for the ground state of helium indicate that the angular dependence of the local contribution to the correlation energy is essentially independent of r1, whereas the local correlation energy shows a strong r1 dependence. The maximum contribution to the correlation energy occurs at intermediate values of γ where there is close agreement between the Hartree–Fock and exact densities.

A new method for the analysis of intramolecular hydrogen bonding is proposed and applied to the equilibrium structures of the protonated fluoroacetones. The method, based on a theory of molecular structure due to Bader, uses the topological properties of the charge density to elucidate the types of interactions within a molecule of interest. The calculations show a strong basis set dependence. In particular, the STO-3G optimized geometries exhibit intramolecular hydrogen bonding between a fluorine substituent and the carbonyl proton, whereas some calculations with geometries optimized at the 4-31G and 6-31G* levels indicate a direct bonding interaction between the fluorine and oxygen atoms. Two types of catastrophe point, namely conflict and bifurcation, are possible in protonated monofluoroacetone. Only the latter was located in the present calculations.

The K–L intershell correlation effects for both singlet and triplet states arising from the 1s2s electronic configuration of the helium atom are analyzed in detail with the aid of the complete (nonreduced) interelectronic probability density and the local Coulomb hole functions. The calculations for both singlet and triplet states suggest the existence of nuclear Coulomb well channels similar to those recently described for the intrashell case of Berry and Krause. When the reference electron is close to the nucleus, the 2 1S Coulomb hole resembles the ground state Coulomb hole, whereas this feature is lost in the 2 3S state due to the effect of exchange correlation. Electron correlation is shown to reduce the nuclear shielding provided by the K‐shell distribution and, as a consequence, to lead to a relative shrinking of the outer shell density. Other differences between the ground state and excited state correlation effects are described.

The topological properties of the electron density and the properties of an atom in a molecule are calculated by means of second-order Moller-Plesset perturbation theory (MP2) and compared with the results of configuration interaction calculations (C12) which include all single and double substitutions from the Hartree-Fock reference configuration. A software package for analyzing the effects of electron correlation on the topological properties of the electron density of molecules is described. H2CO is used to provide a numerical example and to indicate that the number of bond critical points is unaffected by the inclusion of electron correlation. Correlation leads to only a small shift in the positions of bond critical points and a small change in the electron density at bond critical points. It is further shown that the energy of an atom in a molecule can be calculated to an accuracy of 1 kcal/mol and the electron population of an atom to about 0.001e. A statistical method is used to show that the deviation of the MP2 correlation correction relative to the CI2 correlation correction for a variety of atomic properties is about 25%.

Mean values of r 1 and r 12 for the ground and several excited states of the helium isoelectronic sequence are used to demonstrate that a simple scaling which superimposes the distribution function f(r 12) as a function of the atomic number leads to a similar result for the electron density distribution D(r1). On the basis of a screening interpretation of the scaling parameter δ, it is concluded that screening is greater in the singlet than the triplet state of a particular configuration, that screening is greater in the P states than the corresponding S states, and that the screening approaches the limiting value of 1 for the highly excited states. The perturbation expansions of Scherr and Knight are used to determine the limiting value of δ when Z→∞ and the relationship between the scaling parameter and the scale factor, chosen so that a trial wave function satisfies the virial theorem, is discussed. A brief discussion of the scaling of the Coulomb hole is presented.

Ab initio SCF MO calculations for the hydrogen-bonded complexes between nitriles and hydrogen fluoride suggest a strong linear relationship between the charge density at the hydrogen-bond critical point and the hydrogen-bond energy. Further investigation of the topological properties of the charge density indicates that the generalization of the bond-length-bond-order relationship of CC bonds due to Bader et al. may be extended to intermolecular hydrogen bonding. Calculations at the 6–31G ** level, including complete geometry optimization, are reported for the complexes, where R  H, Li, F, Cl, HO, LiO, NC, HCC, CH 3 and CH 3 O.

The radius of the Coulomb hole in an atom or molecule is discussed in general terms and illustrated in detail for the ground state of the helium isoelectronic sequence. The calculations demonstrate a general tendency for the radius of the Coulomb hole to increase as a function of the distance of the reference electron from the nucleus and the existence of a nonzero lower limit (0.800 a.u. in He and 0.111 a.u. in Mg10+) for the radius of the Coulomb hole. This limit is slightly larger than the radial distance at which the radial density distribution reaches its maximum. By expressing the correlation energy of an atom in terms of certain one and two electron expectation values, the relationship between the correlation energy and the size and shape of the Coulomb hole is discussed. Detailed comparisons for the ground and 23S states of He are used to demonstrate the relationship.

An electrostatic model, according to which the electronegativity of an atom is identified with the electrostatic force between the effective nuclear charge and an electron at a distance equal to the relative radius of the atom, is used to calculate a nonempirical electronegativity scale for the first 54 elements. The relative radius is calculated from the free‐atom wave function by use of the density contour approach, while intergration of the radial density from the nucleus to the relative radius yields the effective nuclear charge. In contrast to the empirical methods, the nonempirical electrostatic method is not only applicable to all elements, but treats all elements equally. In addition to correlating well with empirical scales, the proposed model is consistent with the major conclusions of the density functional approach of Parr and his co‐workers. In particular, the model is easily extended to yield valence state electronegativities.

Extensive abinitio molecular orbital calculations are reported for the hydrogen bonded complexes of the formula [Formula: see text] and, [Formula: see text] where m = 0, 1, 2, and 3. The one-proton ("linear") hydrogen bonded complexes of the ammonium ions and hydrogen fluoride are predicted to be more stable by about 1 or 2 kcal/mol than the two- and three-proton ("bent") complexes. Several systematic trends and a number of differences between the neutral and protonated series of complexes are observed. The basis set dependence of the results, basis set superposition errors and electron correlation effects are discussed.

Analytical expansions for the radial density D(r), as calculated within the Hartree–Fock approximation, are reported for the neutral atoms from helium to xenon. As part of a brief discussion of one-electron properties which may be calculated from D(r) or the related density distribution ρ(r), expectation values for rn, n = −2, −1, 1, 2, 3, 4, and 5 are presented.

abinitio Molecular orbital calculations at the STO-3G and STO-3G* levels on H2S2, (CH3)2S2, (CH3CH2)2S2, and L-cystine show that the minimum energy conformation occurs at the angle at which the S—S overlap population is at a maximum. In agreement with experiment, the calculations favour a disulfide dihedral angle close to 90°. The variation of the S—S overlap population is essentially independent of substituents, whereas the cis barrier to rotation about the S—S bond increases relative to the trans barrier as the size of the substituents increases. These observations lead to the conclusion that electronic effects determine the disulfide dihedral angle and that both steric and electronic effects control the relative magnitude of the cis and trans barriers.

A recent proposal for the evaluation of the shape of the Fermi hole in atoms is investigated. It is pointed out that the arbitrariness inherent in the Hartree-Fock orbitals introduces an ambiguity in the definition, which can be overcome by using the unorthogonalized Hartree orbitals. This is still not quite satisfactory, the use of a proper Hartree-Fock as well as a proper Hartree wavefunction, each independently optimized, being essential for obtaining the shape of the Fermi hole. Calculations demonstrating these points are presented for the 23S state of the helium isoelectronic sequence.

The density difference ΔD(r1), defined as the difference between the exact and the Hartree-Fock radial density functions is reported for the 2 3S and the 2 1S excited states of the helium atom. The density differences in both cases resemble the inverse of the helium ground-state density difference, but are more long-ranged due to the diffuseness of the excited states. The effect of electron correlation on one-electron distributions is also demonstrated by comparison of (r1n) expectation values and several indices of the relative sizes of atoms. Electron correlation leads to a contraction of the Hartree-Fock electron clouds.

The topological properties of the charge density of the hydrogen-bonded complexes between nitrites and hydrogen chloride correlate linearly with theoretical estimates of the hydrogen-bond energy. At the 6-31G** level, the hydrogenbond energies range from a low of 10 kJ mol m NCCN—HC1 to a high of 38 kJ mol in LiCN—HCl. A linear relationship between the charge density at the hydrogen-bond critical point and the NH internuclear distance of the RCN—HC1 complexes indicates that the generalization of the bond-length-bond-order relationship of CC bonds due to Bader, Tang, Tal and Biegler-Konig can be extended to intermolecular hydrogen bonding.

Calcul MO ab initio des affinites protoniques de CH n−1 X 4−n − avec n=1 a 4 et X=F, CN ou NO 2 , formiate, fluoroformiate, et d'acetates CH n−1 X 4−n CO 2 − avec n=1 a 4 et X=F, Cl, avec differents ensembles de base que l'on compare

A study of the 1 B 1 and 3 B 1 states of CH2 has been carried out, with the aim of evaluating the singlet–triplet differences in the total energy and its components. These were examined in connection with the interpretation of Hund’s multiplicity rule. Although vertical results near equilibrium tend to vindicate the new explanation of Hund’s rule, the effect of the molecular virial theorem must be considered at large separations. Also, for adiabatic comparisons, the effect of bond lengths on the energy components must be considered. Correlation energy was found to have a significant effect on vertical energy component differences at certain geometries.

The bond critical points of the binary hydrides formed by the elements of the first two rows of the periodic table have been calculated. Particular attention has been paid to the basis-set dependence of the bond critical points at the experimental equilibrium geometries, or where necessary at model geometries. With the exception of H2S, stepwise extension of the basis set leads to a smooth convergence of the bond critical points to a set of values which appear to converge to the Hartree–Fock limit. For H2S it is shownb that the position of the bodn critical point is not only more sensitive to the presence of polarization functions in the basis set, but depends strongly on the orbital exponents of the polarization functions. Extensive optimizations of the exponents of the polarization functions have been carried out with the (12s9p/5s) basis set for second-row hydrides. The effects of contracting the Huzinaga basis sets have been examined.

Ab initio SCF MO calculations are reported for benzene, s -triazine, borazine and boroxine. The Laplacian of the charge density and the Mulliken population analysis procedure demonstrate that the delocalization of the π electrons decreases and the polarity of the ring bonds increases substantially as the atoms in the ring become more dissimilar. Several other properties, including distortion of the ring angles, puckering of the ring and nuclear quadrupolar coupling constants, emphasize the different chemical properties within the isoelectronic series.

A thermochemical cycle is used to estimate the gas phase acidity of several hydrocarbon radical cations: hydrocarbon radical cation (pKa), methane (≥ 86.5 ± 1), ethane(101 ± 1, 102 + 1), ethylene (≥ 125 ± 2), acetylene (118 ± 3, 122 ± 1), propene(124 ± 1, 122 ± 1), cyclopropane (≥ 134, ≥ 131), benzene (147 ± 1), and toluene (139 ± 1, 140). Similarly, proton affinities of the conjugate base, the hydrocarbon radical, are estimated.An estimate of the proton affinity is obtained using abinitio MO calculations. A Hartree–Fock proton affinity, PAHF(R•)g is defined as the difference in the computed energies of a radical cation and its conjugate base at the Hartree–Fock level. Calculations at the SCF level are carried out using both the minimal (STO-3G) and extended basis sets, without (3-21G, 4-31G, and 6-31G) and with (4-31G* and 6-31G*) polarization functions on carbon.The agreement between the thermochemical and abinitio estimates of the proton affinity is satisfactory.

The pair distribution functions evaluated for the 23S state of the helium isoelectronic sequence from the Hart and Herzberg correlated wave functions and those corresponding to the Hartree-Fock approximation are used to determine the shape of the corresponding Coulomb holes. As a consequence of a discontinuity in the Hartree-Fock solution between He and Li+, the Coulomb hole has a different shape for He than for Li+ and the other isoelectronic ions.