Planar Ground State Research Articles

An MO study of neutral C 8 high-symmetry clusters

Highly symmetric octacarbon clusters, i.e. a cube (O h), a twisted cube (T d), a tricyclic cage (C 3v) and a planar (D 4h) structure, were fully optimized at the HF, B3LYP and MP2(Full) levels of theory. These stationary points were shown to be local minima on the potential energy surface by frequency calculations. The stability of these clusters compared to the cyclic planar (C 4h) ground state was found to be strongly dependent on the level of theory. MP2(Full) calculations suggest that cubic C 8 is a local minimum, its energy being 273 kJ/mol higher than the ground state. Structural parameters and fundamental vibrations are presented.

Core ionization energies of amides as a probe of structure and bonding

Core orbital energies are computed for planar ground-state and rotational transition-state structures for formamide and N,N-dimethylacetamide using ab initio molecular orbital calculations at the 6-31G∗ level. Distortion of the amide linkage decreases the core ionization energy of nitrogen and increases the core ionization energies of oxygen and the carbonyl carbon. Similar trends are observed for bridgehead bicyclic lactams and are corroborated by the limited experimental data available. A simple interpretation can be made in the language of resonance theories through reference to contributions of three canonical structures ( 1A–1C) and in particular, the reduced contribution of 1B in distorted amides.

The rotational barrier of an enaminonitrile. Charge and energy redistribution during rotation about the C-N bond

Ab initio quantum mechanical techniques were used together with PROAIM electron density partitioning and CHELPG electrostatic potential analysis to examine the charge density distribution of model enaminonitrile1 in its planar ground state and in its two rotational transition states. The barrier to rotation about the C-N bond was calculated to be 15.4 and 15.6 kcal/mole for the two rotational transition states at the HF/6-31G** level of theory, and was found to originate from a redistribution of electronic kinetic energy between the amino group and the rest of the molecule in a manner similar to that found for formamide and sulfonamide. Similarly, the C-N bond length and amino group electron population were found to depend upon the C-N torsional angle. Electrostatically derived atomic point charges were also examined at each stationary point using the CHELPG program. CHELPG electrostatic potential results were found to represent the traditional “external” viewpoint of the charge density consistent with a resonance model, while the results from PROAIM calculations were found to describe the underlying charge density and kinetic energy density redistribution responsible for the rotational barrier.

Proton transfer equilibrium reactions in donor—acceptor 3H-indole derivatives : Electronic spectroscopy and photophysics of the neutral molecules and their charged species in water

Substituted derivatives of the main chromophore 2-phenyl-3,3-dimethyl-3H-indole [I]have been studied over a wide range of acidity by UV absorption and fluorescence spectroscopy. The spectroscopic and photophysical parameters of the various species have been discussed. The molecules are: 2-[(p-amino) phenyl]-3,3-dimethyl-3H-indole (II), 2-[(p-dimethylamino)phenyl]-3,3-dimethyl-3H-indole (III), 2-[(p-amino)phenyl]-3,3-dimethyl-5-cyano-3H-indole (IV), and 2-[(p-dimethylamino)phenyl]-3,3-dimethyl-5-cyano-3H-indole (V). Spectral characteristics allow us to characterize three species namely the neutral molecule (N), the monocation (C) and the dication (DC) for molecules IV and V. Protonation of the ring nitrogen atom generates a highly planar ground-state cationic species which retains its conformation in the relaxed excited state. Acidity constants have been determined experimentally. A radiationless torsional mechanism is involved in the excited state relaxation of the various species. A low energy non-emissive twisted intramolecular charge transfer (TICT) excited state taking its origin in the amino or anilino groups moiety is suggested from INDO/S calculations to explain the strong fluorescence quenching of the neutral and monocation species in water. It is concluded that molecule V which is the most planar exhibits the strongest TICT excited state formation.

Theoretical study of the Si 3C 2 cluster

Using ab initio calculations based on geometry optimizations at the MP2/DZ2P level various structural and bonding features of the Si 3C 2 system have been investigated. The energies of the MP2 optimized structures are calculated using singles and doubles coupled cluster (CCSD) theory and the CCSD(T) method. The results show that the structure of lowest energy is a C 2v pentagon. This planar structure is stabilized against competing three-dimensional geometries by strong SiC bonds, in accordance with the stability criteria we have suggested earlier. The harmonic frequencies and isotopic shifts of this planar ground state structure are also calculated at the MP2/DZ2P level.

GROUND-STATE ORDERED STRUCTURES OF TERNARY III-V SEMICO-NDUCTOR ALLOYS

摘要 利用了自旋１／２二维Ｉｓｉｎｇ模型及其“不等式方法”，确定了三元Ⅲ─Ｖ半导体会金（００１）面外延生长中的二维平面基态有序结构和基态相图，并通过层状叠加得到了合金中可能出现的长程有序结构，较好地说明了实验发现的（００１）衬底外延生长中的长程有序现象． Abstract Two-dimensional spin 1/2 Ising model and the "inequality method" are used to determine the planar ground-state structures and ground-state phase diagram of ternary III-V semiconductor alloys. Long-range ordered structures in epitaxial growth are deduced according to layer-by-layer stacking. Some experimental long-range ordered features on (001)substrate growth are explained. 作者及机构信息 黄志峰, 倪军, 顾秉林 1. 清华大学现代应用物理系 基金项目: 国家高技术研究发展计划资助的课题. Authors and contacts HUANG ZHI-FENG, NI JUN, GU BING-LIN 1. 清华大学现代应用物理系 参考文献 施引文献

One-, Two- and Three-Dimensional Structures of C20

Abstract The aggregates C20 have been studied by the AM1 quantum-chemical method and at least three different local energy minima found. The located planar cyclic ground state exhibits C 10h symmetry, being followed by two triplet linear structures, a spheroidal C 1 structure, and a bowl-shaped C 5v structure.

Solvent effects on the barrier to isomerization for a tertiary amide from ab initio and Monte Carlo calculations

The effects of solvation on the free energies of activation for rotation about the carbonyl C-N bond in N,N-dimethylacetamide (DMA) have been examined through a combination of gas-phase ab initio calculations and solution-phase statistical mechanics simulations. The geometries of the planar ground state and the transition states with the nitrogen lone anti and syn to the oxygen were optimized with the 6-31G(d) basis set. Subsequent calculations of the vibrational frequencies and correlation energies (MP4(fc)SDTQ/6-31G(d)//6-31G(d)) predict ΔGs of 14.6 and 18.7 kcal/mol for the anti and syn transition states at 298 K

Ground state properties and optical response of LixNa4−x, x=0–4: An a b i n i t i o study

The ground state properties of LixNa4−x (x=0–4) were investigated using extended basis Hartree–Fock calculations. The existence of substitution isomers was found for the mixed alkali clusters and planar singlet ground states were found for all clusters, including the isomers. The optical response of the clusters was modeled via configuration interaction—singles (CIS) in the hope of helping to interpret the experimental data. CIS results were found comparable to experiment at lower excitation energies, but inadequate at higher energies due to the neglect of doubles and higher excitations. Some exploration of the excited state surfaces of Li4 and of one isomer of Li2Na2 is also reported. Comparisons are made between CIS calculations, experiment, and multireference doubly excited configuration interaction calculations.

Optical absorption probes of LixNa4−x (x=1,2): Geometry, substitution isomers, and composition-dependent electron delocalization

Photodepletion spectra (415–763 nm, 10 Å effective resolution) were obtained for LiNa3 and Li2Na2 and compared to previous Na4 and Li4 measurements as well as to model calculations ranging from classical electrostatic to ab initio molecular orbital. Findings include (i) the assignment of planar rhombic singlet ground states for both molecules; (ii) in Li2Na2, the possibleobservation of predominantly one of three energetically close-lying planar rhombic or near rhombic substitution isomers; and (iii) in the sodium/lithium tetramer series, evidence for increasing localization of itinerant valence electrons with increasing cluster lithium content.

Theoretical study of small aluminum phosphide and magnesium sulfide clusters

Electronic structures and stabilities of small AlnPn and MgnSn clusters (n=1–3) are explored by means of accurate quantum chemical calculations. The effects of polarization functions and electron correlation are included in these calculations. Ionic factors are clearly dominant for MgS clusters. Thus, both Mg2S2 and Mg3S3 have planar ground state geometries where charge alternation is utilized effectively. AlP clusters, on the other hand, behave intermediate between the ionic MgS clusters and the covalent Si clusters. Thus, while the ground state structures of Al2P2 and Al3P3 are both analogous to those of the isoelectronic silicon clusters Si4 and Si6, other low-lying minima which are similar to those of MgS clusters are also present. The hybridization and bonding in the different structures are discussed.

Conformational study on alkyl-substituted thiophene oligomers

A series of Austin-method 1 geometry optimizations on alkyl-substituted thiophene small oligomers has been carried out. Torsion potential curves for dimers are obtained as a function of the length of the alkyl side group and the substitution position. Ground-state geometries are predicted to be quite independent of the radical length. On the other hand, different side-chain couplings give rise to distinct torsion potential curves. A planar ground-state geometry is obtained for 4,4′-dialkyl-bithiophene while in 3,3′ and 3,4′ coupled isomers, thiophenes are twisted by 90° but have distinct torsion barriers. The importance of the present results to the structural features of alkyl-substituted polythiophenes is discussed.

Rotational spectra of benzyl cyanide assignment of the planar conformer and evidence of a low barrier to internal rotation

Rotational spectra of benzyl cyanide were obtained at a very low temperature in a pulsed beam Fourier transform microwave spectrometer, and over a range of much higher temperatures in a continuous wave spectrometer. The ground state rotational constants, A = 4346.815 (5) MHz, B = 1057.373(3) MHz and C=856.642(3) MHz, and five centrifugal distortion constants were derived from transitions measured between 7.5 and 17 GHz and are only consistent with a heavy-atom planar structure. The 14N quadrupole coupling constant, X aa = −1.313(2) MHz and X bb = −0.657 (3) MHz, is also only consistent with a planar heavy-atom structure if the electric field gradient in the CN group is assumed cylindrically symmetrical with X zz = −3.9 MHz. Temperature dependent low resolution microwave spectra between 18 and 26.5 GHz display R-branch a-type bands from two types of species. The planar ground state produces a band series with B + C = 1927(2) MHz. At 300 K three series of bands of steadily diminishing intensity are displaced to higher frequency from the ground state bands; they are assigned to three torsionally excited species. Relative intensities of these satellites indicate a low internal rotation barrier. Superposed on this satellite pattern are broader bands of another species whose characteristics are incompatible with another conformational isomer. Its intensity relative to the series assigned to the ground state increases from 0.5 to 4 when the temperature is increased from 280 to 360 K. This series may originate from many weakly bound and/or freely rotating torsional satellites with similar rotational constants.

Spin-glass properties of a planar antiferromagnet

Small-angle neutron scattering is used to define the magnetic state of a powder sample of minnesotaite at points around its hysteresis loop. The silicate [idealized formula [Si4] {Fe2+3}O10(OH)2] has a planar antiferromagentic ground state giving a magnetic 00 (1)/(2) reflection, but this state is destroyed to the profit of a ferromagnetic state (001 reflection) in those grains where the component of the applied field exceeds Hs =0.7 T at 4.2 K, never to be re-established in an isothermal cycle. Likewise, the planar antiferromagnetic ground state is destroyed, and the susceptibility peak eliminated, on field cooling. The existence of spin-glass magnetic properties in a material that does not have a highly degenerate, frustrated ground state derives from the irreversibility of the antiferromagnetic→ferromagnetic transition.

Variational wave function for nonuniform, noncollinear S=1/2 quantum antiferromagnets

The ‘‘Marshall–Huse–Elser’’ variational wave functions describe ordered planar (possibly noncollinear) ground states of s=1/2 Heisenberg spin-exchange Hamiltonians. We show how to generalize these wave functions to allow nonuniform states, arising from interactions which may be random and/or frustrated. In the Szi basis, the amplitude is exp[1/2H̃({Szi})] where the pseudo-Hamiltonian is given by H̃=−∑i2iθiSzi −(1/2)!∑ijKijSziSzj −(1/3!)∑ijkiLijkSzi SzjSzk. Here the classical ground-state directions {θi} (=0 or π in the Néel state) are found by minimizing an effective classical energy F=∑ij[Aij cos(θi−θj) +Bij cos2(θi−θj)], where (Aij,Jij) are functions of nearby Jij’s. Next, Kij and Lijk are taken to be functions of the values {Jij} and the angles {θi−θj}. The functions for Aij, Bij, Kij, and Lijk depend parametrically on a small set of variational parameters. Thus the dimension of parameter space does not grow with system size. We estimate the parameter values analytically, using the spin-wave approximation in a uniformly twisted square-lattice antiferromagnet. Also, the general form of the three-spin coefficient Lijk is roughly a sum of contributions ∝Jij sin(θi−θj), and j and k are both neighbors of spin i.

3P mercury, cadmium, and zinc photosensitized chemistry of vinyl halides in krypton matrix

The reaction of group IIB metals in the {sup 3}P state with vinyl fluoride, chloride, and bromide is studied in krypton matrix. The primary process in all cases is hydrogen halide elimination to form a hydrogen halide/acetylene hydrogen-bonded complex. Insertion of metal atoms into C-Cl and C-Br bonds, but not into C-H and C-F bonds, is also observed. The insertion photochemistry can be explained by a mechanism which requires that the process occur on a triplet surface with the vinyl halide in the planar ground-state conformation.

Molecular structure and relative proton and electron affinities of isomeric nitroimidazoles

The isomeric 2-nitro, 4-nitro, and 5-nitroimidazoles have been studied in their planar ground state, C—NO2 rotational transition state, 3-H protonated conjugate acid and radical anion forms, with abinitio computations at the split-valence 3-21G basis set level. The stabilities of the parent compounds follow the order 5-NO2 ~ 4-NO2 > 2-NO2. In solution 4-nitro is more stable than 5-nitro; the calculations suggest that this is a solvation effect, since the 4-nitro isomer has a considerably higher dipole moment. Barriers for nitro group rotation range from 10 to 16 kcal/mol. However, the relative change in dipole moment during the rotation is small, suggesting that dipole moments of nitroimidazoles are determined mainly by inductive effects, with resonance interactions being relatively unimportant. This conclusion is supported by calculations of molecular electrostatic potentials of the planar and rotated forms. Electron affinities follow the order [Formula: see text], closely matching the order of biological effectiveness of nitroimidazole radiosensitizers. This is consistent with suggestions that the "nitro:nitro radical anion" redox cycle is an important determinant of biological activity. Keywords: nitroimidazole, abinitio computation, rotational barrier, proton affinity, electron affinity.

Photo valence isomerization of sterically strained aromatic hydrocarbons: 8,9-dicarbethoxy[6]paracyclophane

Irradiation of 8,9-dicarbethoxy[6] paracyclophane ( 1) in the long-wavelength band with λ ≈ 310 nm in dilute fluid solutions gives the 1,4-Dewar isomer ( 2) with 2.4% quantum yield. The thermal back reaction 2 → 1 follows an Arrhenius law ( E a = 88.3 ± 3.6 kJ mol -1, log A = 9.3 ± 0.6). By calorimetry an enthalpy of this reaction was determined as δH = -19.8 ± 3.8 kJ mol -1. This is significantly smaller than in unstrained systems with a planar aromatic ground state. No other products were detected in this thermoreversible photoreaction. The photochemical rearomatization 2 → 1 occurs with a quantum yield of 0.12 ± 0.02 with light of λ ≈ 288 nm. Only by irradiation of 2 with λ < 270 nm the prismane isomer ( 3) is formed.

Electronically driven magnetic transition and dimensionality crossover in Li 4 clusters

The existence of a new metastable phase of Li 4 with spin triplet in the shape of a distorted tetrahedron is proposed. It is 0.4 eV above the spin-singlet planar ground state of the tetramer. The transition from the singlet to the triplet configuration does not occur as soon as the cluster assumes a three-dimensional geometry, but waits till the dihedral angle is 110°. Such a dimensionality crossover accompanied by a change in spin multiplicity should be observable.

Chemistry of amido radicals: intramolecular hydrogen abstraction as related to amido radical configurations

Model bromamides were photolysed to investigate the reactivity of intramolecular C-5 hydrogen abstraction (from the nitrogen atom) of the corresponding amidyl radicals and of the cyclization of the resulting C-bromides. The observed results are discussed in terms of stereoelectronic as well as energetic considerations. In the former C-5 hydrogen abstraction, the reactivity difference between semi-rigid trans and cis-oriented reaction centers is only slightly in favor of the latter; the small difference is interpreted as due to the planar ground state Π amidyl radical structure with a low N—CO twisting barrier (4-5 kcal/mol). This interpretation is supported by the efficient C-5 hydrogen abstraction in N-bromo-N-alkyltoluamides, since a rigid planar amidyl radical does not allow the methyl C—H bond to colinearly approach the Π orbital. In marked contrast, there are large reactivity differences in the cyclization of C-bromides derived from these intramolecular H—Br exchanges; these arise from the requirement of colinearity of the approaching and departing bonds. Thus this requires the coplanar orientation of the C—CH3 and C—CO bonds and this is the source of the reactivity differences.