Bond rotation dynamics of enamides: the effect of the acyl group and potential for chirality transfer during 5-endo trig radical cyclizations

Barriers to rotation of the N-alkenyl bond in a series of N-cycloalkenyl-N-benzyl acetamide derivatives have been measured in different solvents by variable-temperature NMR experiments. The barriers range from 9.7 to 14.2 kcal/mol, depending on substituents on the acetamide acyl group. Polar solvents such as chloroform and methanol increase the barrier to rotation compared to nonpolar solvents such as toluene. The barrier to rotation of "mimics" for acetamide-based radicals are estimated. The relative order of the values of k(rot) for different acyl groups parallels their reported Taft E(s) paramaters. For successful chirality transfer in 5-endo trig radical cyclization, it is evident that rotations would need to be significantly slower than those reported here.     

Probing the electronic structure of peptide bonds using methyl groups

The observed V3 torsional barriers measured by microwave spectroscopy for nine methyl groups attached alpha to peptide bond linkages in five gas-phase biomimetics have been found to differ considerably from one molecule to the next and even depend on the position of substitution, being sensitive to structural changes at the other end of the peptide bond. In the search for an explanation for these results, ab initio calculations have been performed at the HF/6-311++G(d,p) level of theory and interpreted in terms of the natural bond orbitals and resonance structures of the peptide bond. These calculations reveal that resonance delocalization in peptide bonds is influenced by methyl conformation through the coupling of vicinal sigma to sigma* orbital interactions with the n to pi*. Thus, CN double-bond character increases (and CO double-bond character decreases) as the methyl group is rotated from the syn to the anti position. A quasilinear correlation exists between the barriers to internal rotation of attached methyl groups and the relative importance of the two principal resonance structures that contribute to the peptide bond.     

Ab initio calculations of the rotational barriers in formamide and acetamide: The effects of polarization functions and correlation

Ab initio calculations have been used to determine the gas-phase rotational barrier about the CN bond in formamide and acetamide. The results indicate that the inclusion of polarization functions in the basis set leads to a substantial decrease (ca. 5 kcal mol⁻¹) in the calculated barrier height at the SCF level. Electron correlation effects decrease the barrier by less than 1 kcal mol⁻¹, while the addition of zero point energy corrections changes the barrier height only slightly. Based upon the current calculations, the 0 K rotational barriers for isolated formamide and acetamide are predicted to be 14.2 and 12.5 kcal mol⁻¹, respectively.     

The interplay of thio(seleno)amide/vinylogous thio(seleno)amide "resonance" and the anisotropic effect of thiocarbonyl and selenocarbonyl functional groups

Amino-substituted thio(seleno)acrylamides 1-4 were synthesized and their 1H and 13C NMR spectra assigned. Both the NMR data and the results of theoretical calculations at the ab initio level of theory were employed to elucidate the adopted structures of the compounds in terms of E/Z isomerism and s-cis/s-trans configuration. In the case of the asymmetrically N(Me)Ph-substituted compounds, ab initio GIAO-calculated ring current effects of the N-phenyl group were applied to successfully determine the preferred conformer bias. The restricted rotations about the two C-N partial double bonds were studied by DNMR and the barriers to rotation (DeltaG(c)++) determined at the coalescence temperatures, and these were discussed with respect to the structural differences between the compounds. The barriers to rotation were also calculated at the ab initio level of theory where the best results (R(2) = 0.8746) were obtained only with inclusion of the solvent at the SCIPCM-HF/6-31G* level of theory. The calculations also provided means of assessing structural influences which were not available due to inaccessible rotation barriers. By means of natural bond orbital (NBO) analysis of 1-4, the occupation numbers of nitrogen lone pairs and bonding/antibonding pi/pi orbitals were shown to quantitatively describe thio(seleno)amide/vinylogous thio(seleno)amide "resonance". Finally, the thio(seleno)carbonyl anisotropic effect was quantitatively calculated by the GIAO method and visualized by isochemical shielding surfaces (ICSS). Only marginal differences between the two anisotropic effects were calculated and are therefore of questionable utility for previous and future applications with respect to stereochemical assignments.     

Excited-state interactions for [Au(CN)(2)(-)](n) and [Ag(CN)(2)(-)](n) oligomers in solution. Formation of luminescent gold-gold bonded excimers and exciplexes.

Solutions of K[Au(CN)(2)] and K[Ag(CN)(2)] in water and methanol exhibit strong photoluminescence. Aqueous solutions of K[Au(CN)(2)] at ambient temperature exhibit luminescence at concentration levels of > or =10(-2) M, while frozen methanol glasses (77 K) exhibit strong luminescence with concentrations as low as 10(-5) M. The corresponding concentration limits for K[Ag(CN)(2)] solutions are 10(-1) M at ambient temperature and 10(-4) M at 77 K. Systematic variations in concentration, solvent, temperature, and excitation wavelength tune the luminescence energy of both K[Au(CN)(2)] and K[Ag(CN)(2)] solutions by >15 x 10(3) cm(-1) in the UV-visible region. The luminescence bands have been individually assigned to *[Au(CN)(2)(-)](n) and *[Ag(CN)(2)(-)](n) excimers and exciplexes that differ in "n" and geometry. The luminescence of Au(I) compounds is related for the first time to Au-Au bonded excimers and exciplexes similar to those reported earlier for Ag(I) compounds. Fully optimized unrestricted open-shell MP2 calculations for the lowest-energy triplet excited state of staggered [Au(CN)(2)(-)](2) show the formation of a Au-Au sigma single bond (2.66 A) in the triplet excimer, compared to a weaker ground-state aurophilic bond (2.96 A). The corresponding frequency calculations revealed Au-Au Raman-active stretching frequencies at 89.8 and 165.7 cm(-1) associated with the ground state and lowest triplet excited state, respectively. The experimental evidence of the exciplex assignment includes the extremely large Stokes shifts and the structureless feature of the luminescence bands, which suggest very distorted excited states. Extended Hückel (EH) calculations for [M(CN)(2)(-)](n) and *[M(CN)(2)(-)](n) models (M = Au, Ag; n = 2, 3) indicate the formation of M-M bonds in the first excited electronic states. From the average EH values for staggered dimers and trimers, the excited-state Au-Au and Ag-Ag bond energies are predicted to be 104 and 112 kJ/mol, respectively. The corresponding bond energies in the ground state are 32 and 25 kJ/mol, respectively.     

Potential Functions for Internal Rotation about the Csp2-O Bond and Intramolecular Interactions in p-RC6H4OCH3 Compounds

Potential functions for internal rotation about the Csp²ÄO bond in p-RC₆H₄OCH₃ compounds (R = NH₂, OCH₃, CH₃, H, F, Cl, CN, NO₂) were determined by nonempirical quantum-chemical calculations in the HF/6-31G* and MP2/6-31G* approximations with account taken of correlation energy for all electrons. The molecular conformation is planar. The height of the rotation barrier changes, depending on the electronic effect of para-substituent. Electron-donor substituents reduce while electron-acceptor substituents enhance the stability of the planar conformation. Using the natural bond orbital (NBO) approach, the nature of lone electron pairs on the methoxy oxygen atom was analyzed, and the energies for their resonance interaction with the antibonding aromatic * orbitals were determined. The effect of para-substituent on the electron density distribution over the methoxy group and on the Koopmans first ionization potentials was estimated. Geometric parameters of the molecules under study are given.     

Experimental studies of light-induced charge transfer and charge redistribution in (X(2)-bipyridine)Re(I)(CO)(3)Cl complexes

Stark emission spectroscopy, transient DC photoconductivity (TDCP), and ground-state dipole moment measurements have been used to evaluate charge transfer (CT) within various (X(2)-bipyridine)Re(I)(CO)(3)Cl complexes following (3)MLCT excited-state formation. The Stark technique reports on vector differences between ground-state (mu(g)) and excited-state (mu(e)) dipole moments, while TDCP, when combined with independently obtained mu(g) information, reports on scalar differences. For systems featuring collinear, same-signed ground- and excited-state dipole moments, the scalar and vector differences are equivalent. However, for the low symmetry systems studied here, they are distinctly different. The vector difference yields the effective adiabatic one-electron-transfer distance (R(12)), while the combined vector and scalar data yield information about dipole rotation upon ground-state/excited-state interconversion. For the systems examined, charge transfer distances are substantially smaller than geometric electron-donor/electron-acceptor site separation distances. The measured distances are significantly affected by changes in acceptor ligand substituent composition. Electron-donating substituents decrease CT distances, while electron-withdrawing substituents increase CT distances, as do aromatic substituents that are capable of expanding the bipyridyl ligand (acceptor ligand) pi system. The Stark measurements additionally indicate that the CT vector and the transition dipole moment are significantly orthogonal, a consequence of strong polarization of the Re-Cl bond (orthogonal to the metal/acceptor-ligand plane) in the ground electronic state and relaxation of the polarization in the upper state. The ground-state Re-Cl bond polarization is sufficiently large that the overall ground-state scalar dipole moment exceeds the overall excited-state scalar dipole moment, despite transfer of an electron from the metal center to the diimine ligand. This finding provides an explanation for the otherwise puzzling negative solvatochromism exhibited in this family of compounds. Combining TDCP and Stark results, we find that the dipole moment can be rotated in some instances by more than 90 degrees upon (3)MLCT excited-state formation. The degree of rotation or reorientation can be modulated by changing the identity of the acceptor ligand substituents. Reorientational effects are smallest when the compounds feature aromatic substituents capable of spatially extending the pi system of the acceptor ligand.     

Anomeric effect in "high energy" phosphate bonds. Selective destabilization of the scissile bond and modulation of the exothermicity of hydrolysis

A natural bonding orbital (NBO) analysis of phosphate bonding and connection to experimental phosphotransfer potential is presented. Density functional calculations with the 6-311++G(d,p) basis set carried out on 10 model phosphoryl compounds verify that the wide variability of experimental standard free energies of hydrolysis (a phosphotransfer potential benchmark) is correlated with the instability of the scissile O-P bond through computed bond lengths. NBO analysis is used to analyze all delocalization interactions contributing to O-P bond weakening. Phosphoryl bond lengths are found to correlate strongest (R = 0.90) with the magnitude of the ground-state n(O) --> sigma*(O-P) anomeric effect. Electron-withdrawing interactions of the substituent upon the sigma(O-P) bonding orbital also correlate strongly with O-P bond lengths (R = 0.88). However, an analysis of sigma*(O-P) and sigma(O-P) populations show that the increase in sigma*(O-P) density is up to 6.5 times greater than the decrease in sigma(O-P) density. Consequently, the anomeric effect is more important than other delocalization interactions in impacting O-P bond lengths. Factors reducing anomeric power by diminishing either lone pair donor ability (solvent) or antibonding acceptor ability (substituent) are shown to result in shorter O-P bond lengths. The trends shown in this work suggest that the generalized anomeric effect provides a simple explanation for relating the sensitivity of the O-P bond to diverse environmental and substituent factors. The anomeric n(O) --> sigma*(O-P) interaction is also shown to correlate strongly with experimentally determined standard free energies of hydrolysis (R = -0.93). A causal mechanism cannot be inferred from correlation. Equally, a P-value of 1.2 x 10(-4) from an F-test indicates that it is unlikely that the ground-state anomeric effect and standard free energies of hydrolysis are coincidentally related. It is found that as the exothermicity of hydrolysis increases, the energy stabilization of the ground-state anomeric effect increases with selective destabilization of the high-energy O-P bond to be broken in hydrolysis. The anomeric effect therefore partially counteracts a larger resonance stabilization of products that makes hydrolysis exothermic and needs to be considered in achieving improved agreement between calculated and empirical energies of hydrolysis. The avenues relating the thermodynamic behavior of phosphates to underlying structural factors via the anomeric effect are discussed.     

Conformational Analysis of Planar Enol Rotamers of 2,4-Pentanedione: an Ab Initio Study

All conformations among different planar enol conformers (rotamers) of 2,4-pentanedione were studied by means of the Hartree-Fock method using the STO-3G** basis set. The calculations were carried out with the Gaussian-98 program. For each conformation, stationary points with the highest energy on the energy curve were found graphically. Several conformations have low energy barriers and correspond to rotations around single bonds. They describe the spatial motion of only one (in most cases, hydrogen) atom or a small molecular fragment. All low energy barriers are in the interval 13-59 kJ·mol^-1. As would be expected, the lowest energy barrier is exhibited by the conformation that leads to the formation of an enol rotamer having an intramolecular H-bond (so-called -shaped form). On the other hand, conformations in which rotation around a bond leads to a break of the intramolecular hydrogen bond have the highest energy barriers. Conformations in which rotation occurs around the double bond have high energy barriers. The influence of the solvents CHCl₃ and CH₃CN on the intramolecular H-bond has also been studied by means of IPCM at the HF/6-31G** level.     

Novel heterometallic Fe-Ru2-Fe arrays via "complex of complexes" approach

Four tetranuclear heterometallic compounds, [(Tp)Fe(CN)3]2[Ru2(DMBA)4] (1), [(MeTp)Fe(CN)3]2[Ru2(DMBA)4] (2), [((i)BuTp)Fe(CN)3]2[Ru2(DMBA)4] (3), and [(PhTp)Fe(CN)3]2[Ru2(DMBA)4] (4) [DMBA = N,N'-dimethylbenzamidinate, Tp = (hydrotris(pyrazolyl)borate, MeTp = (methyltris(pyrazolyl)borate, (i)BuTp = (2-methylpropyltris(pyrazolyl)borate, and PhTp = (tris(pyrazolyl)phenylborate)] were prepared from the combination of Ru2(DMBA)4(NO3)2 and an appropriate [(RTp)Fe(CN)3](-). Molecular structures of compounds 1-4 were established using single-crystal X-ray diffraction, and all feature a linear Fe-C[triple bond]N-Ru-Ru-N[triple bond]C-Fe array. The magnetic study revealed that the temperature dependence of chi(M)T is mostly attributed to the zero-field splitting of the Ru2 center, indicating the absence of strong spin coupling among three metallic centers. The electronic independence was further confirmed by the vis-NIR spectroscopic studies. Also described are the voltammetric properties of these compounds.     

On the stability, electronic structure, and nonlinear optical properties of HXeOXeF and FXeOXeF

The electronic ground state, stability, and linear and nonlinear optical properties of HXeOXeF and FXeOXeF have been studied theoretically by employing complete active space valence bond (CASVB), multistate complete active space perturbation theory (MS-CASPT2), and coupled cluster methods. It is shown that the oxygen inserted between the two Xe atoms significantly modifies the ground-state electronic configuration of the formed derivative by increasing the closed-shell contribution (σ(2)) and removing the diradicaloid character observed in HXe(2)F. The electronic charge distribution has been analyzed by employing the atoms-in-molecules (AIM) method. The dissociation channels of HXeOXeF and FXeOXeF have been studied in detail. It was found that these compounds are metastable, protected by substantial energy barriers and, thus, they can be prepared under appropriate conditions. Both two- and three-body dissociation reactions have been considered. The effects of inserting O in HXe(2)F and substituting H (HXeOXeF) by F, leading to FXeOXeF, on the energy barriers are discussed. The significant effects of the inserted oxygen on the polarizability and even more on the first hyperpolarizability have been computed and rationalized.     

Photodissociation of BrCN and ICN in the a continuum: Vibrational and rotational distributions of CN(X 2Σ+)

We report results of experiments on photodissociation of ICN and BrCN in the long-wavelength (A) continuum in which primary rotational and vibrational distributions of CN in its electronic ground state are determined. For the CN fragment from ICN, our observations at 222 and 249 nm, reported here, together with our earlier results at longer wavelengths, show that the fraction of energy which goes into rotation (in the ground-state channel) is very nearly independent of exciting wavelength in the range 222–351 nm. This holds for each of CN vibrational levels 0, 1, and 2, where the fractions are 15, 25, and 33% respectively. The vibrational distributions decrease monotonously with υ, but show little change as a function of exciting wavelength. CN distributions are also presented for the photodissociation of BrCN at 193, 222, 249, and 308 nm. Results for both parent molecules are briefly discussed in relation to other recent experimental and theoretical work.     

Über die Struktur der Thioamide und ihrer Derivate—XXIII: 14N-Entkoppelte H-NMR-Spektren und Messung der Rotationsbarrieren bei primären Amiden und Thioamiden

By saturation of the ¹⁴N resonance, hindered internal rotation around the CN bond of the (thio)amide system is detected in the H-NMR spectra of the primary amides ( 1a to 1d ) and the thioamides ( 2a to 2g ). With the aid of coupling constants and benzene dilution shifts, it is possible to assign the signals of the amino group to the cis and the trans NH protons. From coalescence results free enthalpies of activation of hindered internal rotation are obtained, and their dependence on steric and electronic effects as well as the influence of the solvent are discussed.     

High-resolution electronic spectrum of the p-difluorobenzene-water complex: structure and internal rotation dynamics

The rotationally resolved S(1) <-- S(0) electronic spectrum of the water complex of p-difluorobenzene (pDFB) has been observed in the collision-free environment of a molecular beam. Analyses of these data show that water forms a planar sigma-bonded complex with pDFB via two points of attachment, a stronger F---H-O hydrogen bond and weaker H---O-H hydrogen bond, involving an ortho hydrogen atom of the ring. Despite the apparent rigidity of this structure, the water molecule also is observed to move within the complex, leading to a splitting of the spectrum into two tunneling subbands. Analyses of these data show that this motion is a combined inversion-internal rotation of the attached water, analogous to the "acceptor-switching" motion in the water dimer. The barriers to this motion are significantly different in the two electronic states owing to changes in the relative strengths of the two hydrogen bonds that hold the complex together.     

ZINDO calculations of the ground state and electronic transitions in the tetracyanonickelate Ion,Ni(CN)(4)(2-)

ZINDO semiempirical calculations on the Ni(CN)(4)(2-) ion were performed, and ground-state energies for all 41 valence-orbital-based MOs and orbital transition components of the two lowest energy fully allowed electronic transitions are reported. Gaussian 94 was used to calculate ground-state energies as a comparison. The ground-state energies using ZINDO compare much more favorably with those found through ab initio techniques than with those from a reported INDO calculation. The found electronic transitions agree substantially with earlier assignments with the exception that several orbital transitions are required to adequately model the lowest energy allowed x,y-polarized experimental transition. Calculation parameters were optimized to give excellent agreement with experiment and may serve well for more complex arrangements of this ion.     

Perfluoroterephthalate bridged complexes with M-M quadruple bonds: ((t)BuCO(2))(3)M(2)(mu-O(2)CC(6)F(4)CO(2))M(2)(O(2)C(t)Bu)(3), where M = Mo or W. Studies of solid-state,molecular, and electronic structure and correlations with electronic and Raman spectral data

The compounds [((t)BuCO(2))(3)M(2)(mu-O(2)CC(6)F(4)CO(2))M(2)(O(2)C(t)Bu)(3)], M(4)PFT, where M = Mo or W, are shown by model fitting of the powder X-ray diffraction data to have an infinite "twisted" structure involving M.O intermolecular interactions in the solid state. The dihedral angle between the M(2) units of each molecule is 54 degrees. Electronic structure calculations employing density functional theory (Gaussian 98 and ADF2000.01, gradient corrected and time dependent) on the model compounds (HCO(2))(3)M(2)(mu-O(2)CC(6)F(4)CO(2))M(2)(O(2)CH)(3), where M = Mo or W, reveal that in the gas phase the model compounds adopt planar D(2)(h) ground-state structures wherein M(2) delta to bridge pi back-bonding is maximized. The calculations predict relatively small HOMO-LUMO gaps of 1.53 eV for M = Mo and 1.22 eV for M = W for this planar structure and that, when the "conjugation" is removed by rotation of the plane of the C(6)F(4) ring to become orthogonal to the M(4) plane, this energy gap is nearly doubled to 2.57 eV for M = Mo and 2.18 eV for M = W. The Raman and resonance Raman spectra of solid M(4)PFT and of Mo(4)PFT in THF solution are dominated by bands assigned to the bridging perfluoroterephthalate (pft) group. The intensities of certain Raman bands of solid W(4)PFT are strongly enhanced on changing the excitation line from 476.5 nm (off resonance) to 676.5 nm, which is on resonance with the W(2) delta --> CO(2) (pft) pi transition at ca. 650 nm. The resonance enhanced bands are delta(s)(CO(2)) (pft) at 518 cm(-)(1) and its first overtone at 1035 cm(-)(1), consistent with the structural change to W(4)PFT expected on excitation from the ground to this pi excited state. The electronic transitions for solid Mo(4)PFT (lowest at 410 nm) were not accessible with the available excitation lines (457.9-676.5 nm), and no resonance Raman spectra of this compound could be obtained. For Mo(4)PFT in THF solution, it is the band at 399 cm(-)(1) assigned to nu(MoMo) which is the most enhanced on approach to resonance with the electronic band at 470 nm; combination bands involving the C(6)F(4) ring-stretching mode, 8a, are also enhanced.     

On the stability of "non-magic" endohedrally doped Si clusters: a first-principles sampling study of MSi16(+) (M = Ti,V,Cr)

Density-functional theory is used to study the geometric and electronic structure of cationic Si(16)(+) clusters with a Ti, V, or Cr dopant atom. Through unbiased global geometry optimization based on the basin-hopping approach, we confirm that a Frank-Kasper polyhedron, with the metal atom at the center, represents the ground-state isomer for all three systems. The endohedral cage geometry is thus stabilized even though only VSi(16)(+) achieves electronic shell closure within the prevalent spherical potential model. Our analysis of the electronic structure traces this diminished role of shell closure for the stabilization back to the adaptive capability of the metal-Si bonding, which is more the result of a complex hybridization than the originally proposed mere formal charge transfer. The resulting flexibility of the metal-Si bond can also help to stabilize "non-magic" cage-dopant combinations, which suggests that a wider range of materials may eventually be cast into this useful geometry for cluster-assembled materials.     

Ab initio vibration—rotation coupling constants and the equilibrium geometries of NCCN and CNCN

The equilibrium geometries of NCCN and CNCN were calculated from experimental ground-state rotational constants and ab initio values for the vibration—rotation coupling constants. For NCCN, R_1e(NC) = 1.1578(5) Å and R_2e(CC) = 1.3839(5) Å were obtained, where estimated error bars are given in parentheses. The calculated equilibrium bond lengths of CNCN are R_1e(CN) = 1.1813(5) Å, R_2e(NC) = 1.3116(5) Å and R_3e(CN) = 1.1581(5) Å. Ground-state rotational and centrifugal distortion constants are predicted with high accuracy for various isotopomers of NCCN.