共查询到20条相似文献,搜索用时 31 毫秒
1.
Maja Nendel Bernd Goldfuss K. N. Houk Klaus Hafner Udo Grieser 《Theoretical chemistry accounts》1999,102(1-6):397-400
B3LYP/6-31G* calculations on bis-periazulene (cyclohepta[def]-fluorene) predict a triplet ground state for this molecule. The singlet has an aromatic 14π-electron periphery but is 2 kcal/mol
higher in energy. The results agree with earlier predictions by Heilbronner.
Received: 19 August 1998 / Accepted: 6 October 1998 / Published online: 23 February 1999 相似文献
2.
High-level ab initio electronic structure theories have been applied to investigate the detailed reaction mechanism of the
spin-forbidden reaction CH(2∏) + N2 → HCN + N(4S). The G2M(RCC) calculations provide accurate energies for the intermediates and transition states involved in the reaction,
whereas the B3LYP/6-311G(d,p) method overestimates the stability of some intermediates by as much as about 10 kcal/mol. A few new structures have been
found for both the doublet and quartet electronic states, which are mainly involved in the dative pathways. However, due to
the higher energies of these structures, the dominant mechanism remains the one involving the C
2 intersystem-crossing step. The C
2 minima on the seam of crossing (MSX) structures and the spin-orbit coupling between the doublet and quartet electronic states
are rather close to those found in previous studies. Vibrational frequencies orthogonal to the normal of the seam which have
been applied in a separate publication to calculate the rate of the CH(2∏) + N2 → HCN + N(4S) reaction with a newly proposed nonadiabatic transition-state theory for spin-forbidden reactions have been calculated at
the MSX from first principles.
Received: 23 June 1998 / Accepted: 21 September 1998 / Published online: 8 February 1999 相似文献
3.
Jeremy N. Harvey Massimiliano Aschi Helmut Schwarz Wolfram Koch 《Theoretical chemistry accounts》1998,99(2):95-99
The phenyl cation is known to have two low-energy minima, corresponding to 1
A
1 and 3
B
1 states, the first of which is more stable by ca. 25 kcal/mol. The minimum energy crossing point between these two surfaces,
located at various levels including a hybrid method first described here, lies just above the minimum of the triplet, 0.12
kcal/mol at the CCSD(T)/cc-pVDZ//B3LYP/SV level, and there is significant spin-orbit coupling between the surfaces at this
point. On the basis of these results, the lifetime of the triplet is expected to be very short.
Received: 24 October 1997 / Accepted: 25 November 1997 相似文献
4.
《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2017,129(34):10317-10321
Tetrakis(trimethylsilyl)cyclobuta‐1,3‐diene ( 1 ) was subjected to a temperature‐dependent EPR study to allow the first spectroscopic observation of a triplet diradical state of a cyclobutadiene ( 2 ). From the temperature dependent EPR absorption area we derive a singlet→triplet ( 1 → 2 ) energy gap, E ST, of 13.9 kcal mol−1, in agreement with calculated values. The zero‐field splitting parameters D =0.171 cm−1, E =0 cm−1 are accurately reproduced by DFT calculations. The triplet diradical 2 is thermally accessible at moderate temperatures. It is not an intermediate in the thermal cycloreversion of cyclobutadiene to two acetylene molecules. 相似文献
5.
Hisayoshi Kobayashi Katsumi Nakashiro Tomoatsu Iwakuwa 《Theoretical chemistry accounts》1999,102(1-6):237-243
The mechanism of ethylene epoxidation on Ag surfaces has been investigated using the density functional method and Ag
n
clusters (n = 3 to 10) modeling the Ag(111) surface. The adsorption energy of O2 to the Ag clusters was strongly dependent on the HOMO level of the cluster, and the clusters with higher HOMO levels afforded
larger O2 adsorption energies. The energetics was investigated for both the molecular and atomic oxygen epoxidation mechanisms. For
the atomic oxygen mechanism, epoxidation was found to proceed without an activation energy, whereas a small amount of activation
energy (about 5 kcal/mol) was calculated for the molecular oxygen mechanism.
Received: 2 July 1998 / Accepted: 9 September 1998 / Published online: 8 February 1999 相似文献
6.
Density functional theory transition structures were located for three concerted [6 + 4] cycloaddition reactions involving
cis-hexatriene and butadiene, cyclopentadiene and cycloheptatriene, and cyclopentadiene and tropone. Geometries, energies, and
entropies were computed at the Becke3LYP/6-31G* level. The activation energy of the concerted [6 + 4] cycloaddition of hexatriene
and butadiene is 33.3 kcal/mol, about 8 kcal/mol above the activation energy of the butadiene plus ethylene [4 + 2] cycloaddition.
The endo concerted [6 + 4] transition state is 1.1 kcal/mol higher than the exo. The [6 + 4] reaction of cyclopentadiene and
cycloheptatriene has a barrier of 25.9 kcal/mol, while the cyclopentadiene–tropone barrier drops to 20.7 kcal/mol.
Received: 3 December 1998 / Accepted: 18 February 1999 / Published online: 7 June 1999 相似文献
7.
We have calculated the free energy differences between four conformers of the linear form of the opioid pentapeptide DPDPE
in aqueous solution. The conformers are Cyc, representing the structure adopted by the linear peptide prior to disulfide bond
formation, β
C
and β
E
, two slightly different β-turns previously identified in unconstrained molecular dynamics simulations, and Ext, an extended
structure. Our simulations indicate that β
E
is the most stable of the studied conformers of linear DPDPE in aqueous solution, with β
C
, Cyc and Ext having free energies higher by 2.3, 6.3, and 28.2 kcal/mol, respectively. The free energy differences of 4.0 kcal/mol
between β
C
and Cyc, and 6.3 kcal/mol between β
E
and Cyc, reflect the cost of pre-organizing the linear peptide into a conformation conducive for disulfide bond formation.
Such a conformational change is a pre-requisite for the chemical reaction of S–S bond formation to proceed. The relatively
low population of the cyclic-like structure agrees qualitatively with observed lower potency and different receptor specificity
of the linear form relative to the cyclic peptide, and with previous unconstrained simulation results. Free energy component
analysis indicates that the moderate stability difference of 4.0–6.3 kcal/mol between the β-turns and the cyclic-like structure
results from cancellation of two large opposing effects. In accord with intuition, the relaxed β-turns have conformational
strain 43–45 kcal/mol lower than the Cyc structure. However, the cyclic-like conformer interacts with water about 39 kcal/mol
strongly than the open β-turns. Our simulations are the first application of the recently developed multidimensional conformational
free energy thermodynamic integration (CFTI) protocol to a solvated system, with fast convergence of the free energy obtained
by fixing all flexible dihedrals. Additionally, the availability of the CFTI multidimensional free energy gradient leads to
a new decomposition scheme, giving the contribution of each fixed dihedral to the overall free energy change and providing
additional insight into the microscopic mechanisms of the studied processes.
Received: 20 April 1998 / Accepted: 9 September 1998 / Published online: 7 December 1998 相似文献
8.
The ground state and several low-lying excited states of the Mg2 dimer have been studied by means of a combination of the complete-active-space multiconfiguration self-consistent-field (CASSCF)/CAS
multireference second-order perturbation theory (CASPT2) method and coupled-cluster with single and double excitations and
perturbative contribution of connected triple excitations [CCSD(T)] scheme. Reasonably good agreement with experiment has
been obtained for the CCSD(T) ground-state potential curve but the dissociation energy of the only experimentally known A1Σ
u
+ excited state of Mg2 is somewhat overestimated at the CASSCF/CASPT2 level. The spectroscopic constants D
e, R
e and ωe deduced from the calculated potential curves for other states are also reported. In addition, some spin–orbit matrix elements
between the excited singlet and triplet states of Mg2 have been evaluated as a function of internuclear separation.
Received: 10 May 2001 / Accepted: 15 August 2001 / Published online: 30 October 2001 相似文献
9.
The accuracy of employing effective core polarization potentials (CPPs) to account for the effects of core-valence correlation
on the spectroscopic constants and dissociation energies of the molecules B2, C2, N2, O2, F2, CO, CN, CH, HF, and C2H2 has been investigated by comparison to accurate all-electron benchmark calculations. The results obtained from the calculations
employing CPPs were surprisingly accurate in every case studied, reducing the errors in the calculated valence D
e values from a maximum of nearly 2.5 kcal/mol to just 0.3 kcal/mol. The effects of enlarging the basis set and using higher-order
valence electron correlation treatments were found to have only a small influence on the core-valence correlation effect predicted
by the CPPs. Thus, to accurately recover the effects of intershell correlation, effective core polarization potentials such
as the ones used in the present work provide an attractive alternative to carrying out computationally demanding calculations
where the core electrons are explicitly included in the correlation treatment.
Received: 11 May 1998 / Accepted: 27 July 1998 / Published online: 28 October 1998 相似文献
10.
Paul G. Wenthold Jun Hu Robert R. Squires W. Carl Lineberger 《Journal of the American Society for Mass Spectrometry》1999,10(9):800-809
The photoelectron spectrum of the trimethylenemethane (TMM) negative ion is described. The electron affinity of TMM is found from the spectrum to be 0. 431±0.006 eV, and the energy difference between the [(X)\tilde]3 A¢2\tilde X^3 A'_2 3A′2 and [(b)\tilde]1 A1\tilde b^1 A_1 1A1 states of TMM is determined to be 16.1±0.2 kcal/mol. The energy difference between the lowest energy triplet and singlet states is estimated to be 13–16 kcal/mol. The enthalpy of formation of TMM is measured to be 70±3 kcal/mol, and the C-H bond enthalpy in 2-methylallyl radical is 90±2 kcal/mol. Previously unobserved vibrational frequencies of 425, 915, and 1310 cm−1 are found for the triplet state of TMM, whereas a frequency of 325 cm−1 is found for the singlet state. In addition, an overtone peak is observed for the triplet state at 1455 cm−1, and both states contain peaks that are assigned to bands arising from excited vibrational levels of the ion. 相似文献
11.
Xianyang Chen Chongde Li Tao Wu Tianyang Yao Guanzhi Ju 《Theoretical chemistry accounts》1998,99(4):272-276
Ab initio molecular electronic structure methods have been used to study the relative stability of the planar inorganic ring
(HAlNH)n (n = 2–4) during homodesmotic and monomer polymerization reactions. Optimized geometries, frequencies and energies through
restricted Hartree-Fock/6-31G* are reported, and energies at the self-consistent field optimized geometries including M?ller-Plesset
fourth perturbation theory with single, double and quadruple excitation (MP4SDQ) corrections are also reported for both reactions.
Homodesmotic reactions with MP4SDQ −28.5 kcal/mol for (AlN)2, 1.9 kcal/mol for (AlN)3 and −0.97 kcal/mol for (AlN)4. On analysing a π-molecular orbitals diagram, only one, three and three strongly bonding π-molecular orbitals exist for the
planar four-, six- and eight-membered AlN rings, respectively.
Received: 9 March 1998 / Accepted: 19 March 1998 / Published online: 23 June 1998 相似文献
12.
Observation of a Thermally Accessible Triplet State Resulting from Rotation around a Main‐Group π Bond
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Arseni Kostenko Dr. Boris Tumanskii Dr. Miriam Karni Prof. Shigeyoshi Inoue Prof. Masaaki Ichinohe Prof. Akira Sekiguchi Prof. Yitzhak Apeloig 《Angewandte Chemie (International ed. in English)》2015,54(41):12144-12148
We report the first direct spectroscopic observation by electron paramagnetic resonance (EPR) spectroscopy of a triplet diradical that is formed in a thermally induced rotation around a main‐group π bond, that is, the Si?Si double bond of tetrakis(di‐tert‐butylmethylsilyl)disilene ( 1 ). The highly twisted ground‐state geometry of singlet 1 allows access to the perpendicular triplet diradical 2 at moderate temperatures of 350–410 K. DFT‐calculated zero‐field splitting (ZFS) parameters of 2 accurately reproduce the experimentally observed half‐field transition. Experiment and theory suggest a thermal equilibrium between 1 and 2 with a very low singlet–triplet energy gap of only 7.3 kcal mol?1. 相似文献
13.
The low-lying singlet and triplet states of H2CBe and HCBeH are examined using ab inito molecular orbital theory. In agreement with earlier results, the lowest-lying structure of H2CBe has C2v symmetry and is a triplet with one π electron (3 B1). The results presented here suggest that the lowest-energy singlet structure is the (1B1) open-shell singlet, also with C2v symmetry, at least 2.5 kcal/mol higher in energy. The singlet C2v structure with two π electrons (1A1) is 15.9 kcal/mol higher than 3B1. All of these structures are bound with respect to the ground state of methylene and the beryllium atom. In HCBeH, linear equilibrium geometries are found for the triplet (3Σ) and singlet (1Δ) states. The triplet is more stable than the singlet (1Δ) by 35.4 kcal/mol, and is only 2.9 kcal/mol higher in energy than triplet H2 CBe. Since the transition structure connecting these two triplet molecules is found to be 50.2 kcal/mol higher in energy than H2 CBe, both triplet equilibrium species might exist independently. The harmonic vibrational frequencies of all structures are also reported. 相似文献
14.
Manabu Sugimoto Ichiro Yamasaki Nobuteru Mizoe Masaharu Anzai Shigeyoshi Sakaki 《Theoretical chemistry accounts》1999,102(1-6):377-384
Acetylene insertion into Pt(II)–H and Pt(II)SiH3 bonds of PtH(SiH3)(PH3) was investigated using ab initio molecular orbital and M?ller-Plesset perturbation theory methods. The insertion into PtH
was predicted to proceed with a smaller activation energy (E
a
=12.8 kcal/mol) than that into PtSiH3 (E
a
=20.9 kcal/mol). The reaction energy (ΔE) of the insertion into PtH is 10 kcal/mol smaller than that for the insertion into PtSiH3, which reflects differences in bond energies between CH and CSi and between PtH and PtSiH3. A comparison with ethylene insertion revealed that the acetylene insertion occurs more easily, and the latter reaction is
more exothermic. A simple vibronic coupling model combined with Toyozawa's interaction mode analysis was used to examine interesting
differences in E
a
and ΔE between insertions into PtH and PtSiH3, and between acetylene and ethylene insertions. This analysis suggests that the factors determining E
a
are the stiffness of the PtH and PtSiH3 bonds and the vibronic coupling strength of acetylene and ethylene.
Received: 13 August 1998 / Accepted: 2 September 1998 / Published online: 15 February 1999 相似文献
15.
Nobuaki Koga 《Theoretical chemistry accounts》1999,102(1-6):285-292
Ethylene insertion into the Sm–C bond of H2SiCp2SmCH3, a model reaction of an olefin polymerization propagation step, has been studied by ab initio molecular orbital methods.
The small electronegativity of the Sm atom makes the Sm–C bond ionic, the methyl group being negatively charged by −0.75.
The reaction passes through a loose ethylene complex with a binding energy of 15 kcal/mol and then a tight four-centered transition
state with an agostic interaction between the Sm atom and one of the methyl CH bonds. A small activation energy of 14 kcal/mol
is required to pass through this transition state, indicating that this is an easy reaction. Compared with the reactions with
group 4 cationic silylene-bridged metallocenes the activation energy is higher and the reaction is less exothermic. The origin
of these differences is discussed. The results of molecular mechanics calculations on regio- and stereoselectivities in the
insertion reaction of propylene are also reported.
Received: 13 July 1998 / Accepted: 28 August 1998 / Published online: 2 November 1998 相似文献
16.
Yukio Kawashima Tomohiro Hashimoto Haruyuki Nakano Kimihiko Hirao 《Theoretical chemistry accounts》1999,102(1-6):49-64
The valence π → π
* excited states of anthracene and naphthacene are studied with multireference perturbation theory with complete active space
self-consistent field reference functions. The predicted spectra provide a consistent assignment of all one- and two-photon
spectra and T-T spectra of low-lying valence π → π
* excited states of anthracene and naphthacene. The present theory predicts the valence π → π
* excitation energies with an accuracy of 0.15 eV for anthracene and of 0.25 eV or better for naphthacene. The excited states
of anthracene and naphthacene are compared with those of benzene and naphthalene studied previously. The present calculations
predict that, going from anthracene to naphthacene, there is a symmetry reversal of the two lowest singlet state transitions,
but not for the triplet, just as indicated by the experimental data. Some general trends of polyacene excited states are discussed
based on the calculated results for benzene to naphthacene. Conclusive results obtained for anthracene and naphthacene can
be used as a model for understanding the excited states of larger polyacenes.
Received: 22 April 1998 / Accepted: 6 July 1998 / Published online: 28 September 1998 相似文献
17.
The reaction between triplet methylene and nitric oxide, producing the formaldiminoxy (CH2NO) radical, and the subsequent decomposition and isomerization reactions of CH2NO have been studied using ab␣initio quantum chemical techniques that include the Gaussian-2 (G2), CASSCF and CASPT2 methods.
Stationary points on the potential energy surfaces were located at MP2/6-31G(d) and CASSCF/cc-pVDZ levels of theory, while the electronic energies were determined using G2, G2(MP2), QCISD(T)/cc-pVTZ,
RCCSD(T)/cc-pVTZ and CASPT2/cc-pVTZ approaches. G2 is believed to be reliable at equilibrium geometries, but the determination
of certain transition state geometries and energies requires a MCSCF-based approach. The calculations suggest that CH2NO (2A′) forms in a barrierless reaction and could readily decompose to H+HCNO. A subsequent abstraction reaction then results in
H2+CNO. No molecular elimination channel was found. An alternative pathway is the formation of CH2ON, which readily isomerizes to CH2NO.
Received: 8 May 1998 / Accepted: 11 August / Published online: 9 October 1998 相似文献
18.
Generation of singlet and triplet 2-silylcyclopentane-1,3-diyls and their reactivity have been investigated in the thermal and photochemical denitrogenation of 2,3-diaza-7-silylbicyclo[2.2.1]hept-2-ene. 5-Silylcyclopentene (silyl migration product) is quantitatively obtained, while 5-silylbicyclo[2.1.0]pentane (intramolecular ring-closure product) is not detected in the denitrogenation reactions. Deuterium labeling studies clarify that 5-silylcyclopentene is formed by a suprafacial [1,2] silyl migration in singlet 2-silylcyclopentane-1,3-diyl. UDFT calculations closely reproduce the observed reactivity of the singlet diradical: The enthalpic barriers of the intramolecular ring-closure are calculated to be DeltaH++exo468 = 5.8 kcal/mol and DeltaH++endo468 = 6.7 kcal/mol, which are much higher than the energy barrier for the [1,2] silyl migration, DeltaH++468 = 2.7 kcal/mol. The notable effect of the silyl group on raising the energy barrier of the intramolecular cyclization is rationalized by an electronic configuration of the lowest singlet state of 2-silylcyclopentane-1,3-diyls. 相似文献
19.
Kizashi Yamaguchi Satoshi Yabushita Takayuki Fueno Shigeki Kato Keiji Morokuma 《Chemical physics letters》1980,70(1):27-30
Geometry optimizations of several low-lying diradical states of the ring-opened oxirane (·CH2CH2O·) were performed by using the energy gradients of the UHF MINDO/3, STO-3G and 4-31G solutions. Both the STO-3G and 4-3 IG methods predict that the most stable form is the triplet state of the non-twisted σπ conformation in which the unpaired spins localized on the terminal carbon and oxygen atoms are oriented perpendicularly to each other. The singlet σσ diradical state in which both the radical-site p orbitals are coplanar with the molecular framework is only 2.3 (STO-3G) and 1.2 (4-31G) kcal/mol less stable than the triplet σπ diradical state. It is found that the geometry of the singlet σσ diradical is unique in that the C-C-O angle is noticeably small as compared with various other diradical states. Implications of these results to the mechanism of the oxirane-forming O(3P) + C2H4 reaction are discussed. 相似文献
20.
Using a recently proposed orbital deletion procedure and the block-localized wavefunction method, the rotational barriers
in H2BNH2 and H2BPH2 are analyzed in terms of conjugation, hyperconjugation, steric effect and pyramidalization. With the zero-point energy corrections,
the π-binding strengths in the planar H2BNH2 and H2BPH2 are both around 20 kcal/mol at the HF level using the 6-311+G** basis set. With the deactivation of the π atomic orbitals
on the boron atom and the evolution from a planar structure to a 90°-twisted structure, the steric repulsion between the B‐H
and the N‐H or P‐H is relieved and moreover, the negative hyperconjugation from the lone electron pair or pairs on the nitrogen
or phosphorus atoms to the antibonding orbital χ*
B
H
2 of the BH2 group stabilizes the twisted structure by 7.4(8.8) or 4.0(5.0) kcal/mol at the HF/6-31G*(6-311+G**) level. However, the repulsive
interaction between the lone pair(s) and the two BH σ bonds is so prominent that the overall steric effect contributes 20.3(22.9)
and 19.3(19.8) kcal/mol to the rotational barriers in H2BNH2 and H2BPH2 with the 6-31G*(6-311+G**) basis set. The present techniques and analyses may also give some clues to justify the parameterization
in the empirical molecular mechanics methods.
Received: 17 April 1998 / Accepted: 17 September 1998 / Published online: 1 February 1999 相似文献