Conformational stability, optimized geometries, vibrational and electronic spectra of methacryloyl bromide in ground and excited electronic states

In order to understand conformational isomerism in methacryloyl bromide (MABR) in the ground (S(0)) and the first excited (S(1)) electronic states and to interpret the vibrational and electronic spectra of its conformers in the S(0) state, quantum mechanical calculations using Density Functional Theory (DFT) and RHF methods with extended basis sets 6-31G, 6-31G** and 6-311+G(d,p) have been conducted. In RHF calculations, electron correlation effects have been included at the M?ller-Plesset MP2 level. It is inferred that in both the electronic states the molecule may exist in two isomeric forms-s-trans and s-cis; the former being more stable than the later by about 1.629 kcal mol(-1) in the S(0) state and by about 2.218 kcal mol(-1) in the S(1) state. Electronic transition tends to increase the s-trans/s-cis and s-cis/s-trans, rotational barriers from 7.059 kcal mol(-1) (2468.1 cm(-1)) and 5.428 kcal mol(-1) (1897.8 cm(-1)) in S(0) state to 23.594 kcal mol(-1) (8249.4 cm(-1)) and 21.376 kcal mol(-1) (7473.9 cm(-1)) in the S(1) state. Completely optimized geometries of the two conformers in S(0) state reveal that while there is no significant difference in their bond lengths, some of the bond angles associated with COBr group are appreciably different. Electronic excitation tends to change both the bond lengths and bond angles. Based on suitably scaled DFT and RHF results obtained from the use of 6-31G** and 6-311+G(d,p) basis sets, a complete assignment is provided to the fundamental vibrational bands of both the s-trans and s-cis conformers in terms of frequency, form and intensity of vibrations and potential distribution across the symmetry coordinates in the S(0) state and a comparison has been made with experimental assignments. A theoretical prediction of the electronic transitions in the near UV-region in the two conformers and their tentative assignment has been provided on the basis of CI level calculations using 6-31G basis set.     

N-H···π hydrogen-bonding and large-amplitude tipping vibrations in jet-cooled pyrrole-benzene

The N-H···π hydrogen bond is an important intermolecular interaction in many biological systems. We have investigated the infrared (IR) and ultraviolet (UV) spectra of the supersonic-jet cooled complex of pyrrole with benzene and benzene-d(6) (Pyr·Bz, Pyr·Bz-d(6)). DFT-D density functional, SCS-MP2 and SCS-CC2 calculations predict a T-shaped and (almost) C(s) symmetric structure with an N-H···π hydrogen bond to the benzene ring. The pyrrole is tipped by ω(S(0)) = ±13° relative to the surface normal of Bz. The N···ring distance is 3.13 ?. In the S(1) excited state, SCS-CC2 calculations predict an increased tipping angle ω(S(1)) = ±21°. The IR depletion spectra support the T-shaped geometry: The NH stretch is redshifted by -59 cm(-1), relative to the "free" NH stretch of pyrrole at 3531 cm(-1), indicating a moderately strong N-H···π interaction. The interaction is weaker than in the (Pyr)(2) dimer, where the NH donor shift is -87 cm(-1) [Dauster et al., Phys. Chem. Chem. Phys., 2008, 10, 2827]. The IR C-H stretch frequencies and intensities of the Bz subunit are very similar to those of the acceptor in the (Bz)(2) dimer, confirming that Bz acts as the acceptor. While the S(1)←S(0) electronic origin of Bz is forbidden and is not observable in the gas-phase, the UV spectrum of Pyr·Bz in the same region exhibits a weak 0 band that is red-shifted by 58 cm(-1) relative to that of Bz (38?086 cm(-1)). The origin appears due to symmetry-breaking of the π-electron system of Bz by the asymmetric pyrrole NH···π hydrogen bond. This contrasts with (Bz)(2), which does not exhibit a 0 band. The Bz moiety in Pyr·Bz exhibits a 6a band at 0 + 518 cm(-1) that is about 20× more intense than the origin band. The symmetry breaking by the NH···π hydrogen bond splits the degeneracy of the ν(6)(e(2g)) vibration, giving rise to 6a' and 6b' sub-bands that are spaced by ～6 cm(-1). Both the 0 and 6 bands of Pyr·Bz carry a progression in the low-frequency (10 cm(-1)) excited-state tipping vibration ω', in agreement with the change of the ω tipping angle predicted by SCS-MP2 and SCS-CC2 calculations.     

Structure of 7-azaindole···2-fluoropyridine dimer in a supersonic jet: competition between N-H···N and N-H···F interactions

In the present work, we have investigated the structure of 7-azaindole···2-fluoropyridine dimer in a supersonic jet by employing resonant two photon ionization (R2PI), IR-UV, and UV-UV double resonance spectroscopic techniques combined with quantum chemistry calculations. The R2PI spectrum of the dimer is recorded by electronic excitation of the 7-azaindole moiety, and a few low frequency intermolecular vibrations of the dimer are clearly observed in the spectrum. The electronic origin band of the dimer is red-shifted by 1278 cm(-1) from the S(1) ← S(0) origin band of 7-azaindole monomer. The presence of a single conformer of the dimer is confirmed by IR-UV and UV-UV hole-burning spectroscopic techniques. RIDIR (Resonant ion dip infrared) spectrum of the dimer shows a red-shift of 265 cm(-1) in the N-H stretching frequency with respect to that of the 7-azaindole monomer. Two planar double hydrogen bonded cyclic structures of the dimer have been predicted from DFT calculations. Comparison of experimental and theoretical N-H stretching frequencies confirms that the observed dimer is stabilized by N-H···N and C-H···N hydrogen bonding interactions. The less stable conformer with N-H···F and C-H···N interactions are not observed in the experiment. The competition between N-H···N and N-H···F interactions in the two dimeric structures are discussed from natural bond orbital (NBO) analysis. The current results demonstrate that fluorine makes a hydrogen bond of intermediate strength through cooperative interaction of another hydrogen bond (C-H···N) present in the dimer, although fluorine is believed to be very weak hydrogen bond acceptor.     

High-resolution laser spectroscopy on the S1<--S0 transition of jet-cooled anthracene: rotational structure and Stark effect

We present rotationally resolved spectra of the S(1)<--S(0) transition of anthracene at 27,687.153(4) cm(-1) as well as Stark effect measurements of the free anthracene molecule in electric fields of up to 85 kV/cm. The molecule is rotationally cooled in a supersonic jet expansion to a temperature of 4 K. The rotational constants of the electronic states S(0) and S(1) are determined by a simplex fit comparing the experimental spectra with simulations for an asymmetric rigid rotor. The measured and simulated energies are in very good agreement and the estimated accuracy of the rotational constants is 1 per thousand. Furthermore, the polarizabilities of the electronic states S(0) and S(1) are investigated. At an electric field of 85 kV/cm, line shifts of up to 150 MHz caused by a change in the polarizability of Deltaalpha=123(7) a.u. and broadenings due to the anisotropy are observed. The components of the tensor polarizabilities of the electronic states S(0) and S(1) are determined by simulating the complete spectra using second-order perturbation theory.     

Comparative density functional theory and post-Hartree-Fock (CCSD, CASSCF) studies on the electronic structure of halogen nitrites ClONO and BrONO using quantum chemical topology

In this paper, the electronic structures of cis- and trans-ClONO and BrONO are studied at the CCSD∕aug-cc-pVTZ, CASSCF(14,12)/aug-cc-pVTZ, and B3LYP/aug-cc-pVTZ computational levels. For the Cl-O bond, topological analysis of the electron density field, ρ(r), shows the prevalence of the shared-electron type bond (?(2)ρ((3,-1)) < 0). The Br-O bond, however, represents the closed-shell interaction (?(2)ρ((3,-1)) > 0). Topological analysis of the electron localization function, η(r), and electron localizability indicator (ELI-D), (D) (σ)(r), shows that the electronic structure of the central N-O bond is very sensitive to both electron correlation improvements (coupled-cluster single double (CCSD), CASSCF, density functional theory (DFT)) and bond length alteration. Depending on the method used, the N-O bond can be characterized as a "normal" N-O bond with a disynaptic V(N,O) basin (DFT); a protocovalent N-O bond with two monosynaptic, V(N) and V(O), basins (CCSD, CASSCF); or a new type, first discovered for FONO, characterized by a single monosynaptic, V(N) basin (CCSD, DFT). The total basin population oscillates between 0.46-0.96 e (CCSD) and 0.86-1.02 e (CASSCF). The X-O bond is described by the single disynaptic basin, V(X,O), with a basin population between 0.76 and 0.81 e (CCSD) or 0.77 and 0.85 e (CASSCF). Analysis of the localized electron detector distribution for the cis-Cl-O1-N=O2 shows a manifold in the Cl···O2 region, associated with decreased electron density.     

Inhibition of light-induced tautomerization of 7-azaindole by phenol: indications of proton-coupled electron/energy transfer quenching

The photophysical behavior of a 1:1 complex between phenol and 7-azaindole (7AI) has been investigated in methylcyclohexane solutions at temperatures in the range of 27 to -50 °C. A linear Benesi-Hildebrand plot associated with changes in absorbance of the complex with phenol concentration in the solutions ensures 1:1 stoichiometry of the produced complex. Our estimate for the value of the association constant (K(a)) of the complex is ~120 M?1 at 27 °C, and it is nearly twice compared to that for 1:1 complex between 7AI and ethanol measured under the same condition. The complexation results in dramatic quenching of the normal fluorescence of 7AI and the process is accelerated upon lowering of temperature. The measured spectra show no indication that phenol promotes tautomerization of 7AI in the excited state. We have argued that the hydrogen bonding between pyridinic N and phenolic O-H (N···O-H) is a vital structural factor responsible for quenching of 7AI fluorescence, and this idea has been corroborated by showing that under same condition the fluorescence of 7AI is enhanced in the presence of anisole. As a plausible mechanism of quenching, we have invoked a proton-coupled electron transfer (PCET) process between phenol and excited 7AI, which outweighs the competing tautomerization process. An analysis in terms of Remm-Weller model reveals that the PCET process involving phenol and excited 7AI could be energetically favorable (ΔG(ET)(0) < 0). An alternative mechanism, where quenching can occur via electronic energy transfer from the excited protonated 7AI to phenoxide ion, following a proton transfer along the N···O-H hydrogen bond, is also discussed.     

Highly luminescent half-lantern cyclometalated platinum(II) complex: synthesis, structure, luminescence studies, and reactivity

The half-lantern compound [{Pt(bzq)(μ-C(7)H(4)NS(2)-κN,S)}(2)]·Me(2)CO (1) was obtained by reaction of equimolar amounts of potassium 2-mercaptobenzothiazolate (KC(7)H(4)NS(2)) and [Pt(bzq)(NCMe)(2)]ClO(4). The Pt(II)···Pt(II) separation in the neutral complex [{Pt(bzq)(μ-C(7)H(4)NS(2)-κN,S)}(2)] is 2.910 (2) ?, this being among the shortest observed in half-lantern divalent platinum complexes. Within the complex, the benzo[h]quinoline (bzq) groups lie in close proximity with most C···C distances being between 3.3 and 3.7 ?, which is indicative of significant π-π interactions. The reaction of 1 with halogens X(2) (X(2) = Cl(2), Br(2), or I(2)) proceeds with a two-electron oxidation to give the corresponding dihalodiplatinum(III) complexes [{Pt(bzq)(μ-C(7)H(4)NS(2)-κN,S)X}(2)] (X = Cl 2, Br 3, I 4). Their X-ray structures confirm the retention of the half-lantern structure and the coordination mode of the bzq and the bridging ligand μ-C(7)H(4)NS(2)-κN,S. The Pt-Pt distances (Pt-Pt = 2.6420(3) ? 2, 2.6435(4) ? 3, 2.6690(3) ? 4) are shorter than that in 1 because of the Pt-Pt bond formation. Time dependent-density functional theory (TD-DFT) studies performed on 1 show a formal bond order of 0 between the metal atoms, with the 6p(z) contribution diminishing the antibonding character of the highest occupied molecular orbital (HOMO) and being responsible for an attractive intermetallic interaction. A shortening of the Pt-Pt distance from 2.959 ? in the ground state S(0) to 2.760 ? in the optimized first excited state (T(1)) is consistent with an increase in the Pt-Pt bond order to 0.5. In agreement with TD-DFT calculations, the intense, structureless, red emission of 1 in the solid state and in solution can be mainly attributed to triplet metal-metal-to-ligand charge transfer ((3)MMLCT) [dσ*(Pt-Pt) → π*(bzq)] excited states. The high quantum yields of this emission measured in toluene (44%) and solid state (62%) at room temperature indicate that 1 is a very efficient and stable (3)MMLCT emitter, even in solution. The high luminescence quantum yield of its red emission, added to its neutral character and the thermal stability of 1, make it a potential compound to be incorporated as phosphorescent dopant in multilayer organic light-emitting devices (OLEDs).     

The S1/S2 exciton interaction in 2-pyridone·6-methyl-2-pyridone: Davydov splitting, vibronic coupling, and vibronic quenching

The excitonic splitting between the S(1) and S(2) electronic states of the doubly hydrogen-bonded dimer 2-pyridone[middle dot]6-methyl-2-pyridone (2PY·6M2PY) is studied in a supersonic jet, applying two-color resonant two-photon ionization (2C-R2PI), UV-UV depletion, and dispersed fluorescence spectroscopies. In contrast to the C(2h) symmetric (2-pyridone)(2) homodimer, in which the S(1) ← S(0) transition is symmetry-forbidden but the S(2) ← S(0) transition is allowed, the symmetry-breaking by the additional methyl group in 2PY·6M2PY leads to the appearance of both the S(1) and S(2) origins, which are separated by Δ(exp) = 154 cm(-1). When combined with the separation of the S(1) ← S(0) excitations of 6M2PY and 2PY, which is δ = 102 cm(-1), one obtains an S(1)/S(2) exciton coupling matrix element of V(AB, el) = 57 cm(-1) in a Frenkel-Davydov exciton model. The vibronic couplings in the S(1)/S(2) ← S(0) spectrum of 2PY·6M2PY are treated by the Fulton-Gouterman single-mode model. We consider independent couplings to the intramolecular 6a(') vibration and to the intermolecular σ(') stretch, and obtain a semi-quantitative fit to the observed spectrum. The dimensionless excitonic couplings are C(6a(')) = 0.15 and C(σ(')) = 0.05, which places this dimer in the weak-coupling limit. However, the S(1)/S(2) state exciton splittings Δ(calc) calculated by the configuration interaction singles method (CIS), time-dependent Hartree-Fock (TD-HF), and approximate second-order coupled-cluster method (CC2) are between 1100 and 1450 cm(-1), or seven to nine times larger than observed. These huge errors result from the neglect of the coupling to the optically active intra- and intermolecular vibrations of the dimer, which lead to vibronic quenching of the purely electronic excitonic splitting. For 2PY·6M2PY the electronic splitting is quenched by a factor of ~30 (i.e., the vibronic quenching factor is Γ(exp) = 0.035), which brings the calculated splittings into close agreement with the experimentally observed value. The 2C-R2PI and fluorescence spectra of the tautomeric species 2-hydroxypyridine·6-methyl-2-pyridone (2HP·6M2PY) are also observed and assigned.     

Persistence rewarded: successful observation of the B 2Sigmau+-X 2Pig electronic transition of jet-cooled BS2

The B-X electronic transition of jet-cooled BS2 has been observed using laser-induced fluorescence techniques. The boron disulfide radical was produced in a pulsed electric discharge jet using a mixture of BCl3 and CS2 in high-pressure argon as the precursor. The spectrum consists of a strong 0(0)(0) band with the 2Sigma-2Pi(3/2) component at 24,393.2 cm(-1) and short progressions in the symmetric stretching (nu1' = 506.7 cm(-1)) and bending (nu2' = 303.2 cm(-1)) modes. A rotational analysis of both spin-orbit components of the 0(0)(0) band gave an upper state B value of 0.0932779(19) cm(-1) and a ground-state spin-orbit coupling constant of A = -405.163(4) cm(-1). The ground-state bond length of 1.66492 angstroms increases to 1.6812(1) angstroms on sigmau --> pig electronic excitation. The B-X data have been used to further refine the Renner-Teller analysis, which is in good agreement with our previous work [J. Chem. Phys. 119, 2047 (2003)].     

Experimentally measured permanent dipoles induced by hydrogen bonding. The Stark spectrum of indole-NH3

Hydrogen bond pairs involving the chromophore indole have been extensively studied in the gas phase. Here, we report high resolution electronic spectroscopy experiments on the indole-NH(3) hydrogen bond pair in the absence and presence of an electric field. The S(1)-S(0) origin band of this complex recorded in zero field at high resolution reveals two overlapping spectra; a consequence of NH(3) hindered internal rotation. The barrier to internal rotation is predicted by theory to be less than 20 cm(-1) in the ground state, therefore requiring a non-rigid rotor Hamiltonian to interpret the spectra. Conducting the experiment in the presence of an applied electric field further perturbs the already congested spectrum of the complex, but makes possible the measurement of the permanent electric dipole moments in its S(0) and S(1) states. These values reveal significant changes in electron distribution that arise from hydrogen bonding effects.     

Pyrophosphate-mediated magnetic interactions in Cu(II) coordination complexes

The reaction in water of Cu(NO(3))(2)·2.5H(2)O with 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen), or 1,10-phenanthroline-5-amine (phenam), and sodium pyrophosphate (Na(4)P(2)O(7)), at various pHs, afforded three new copper(II)-pyrophosphate complexes, namely, {[Cu(bipy)(cis-H(2)P(2)O(7))](2)}·3H(2)O (1a), {[Cu(phen)(H(2)O)](4)(HP(2)O(7))(2)}(ClO(4))(2)·4H(2)O (2), and {[Cu(2)(phenam)(2)(P(2)O(7))](2)·25H(2)O}(n) (3). A solvent free crystalline phase of 1a was also isolated with formula {[Cu(bipy)(trans-H(2)P(2)O(7))](2)} (1b), which can be regarded as a pseudo-polymorph of 1a. Single crystal X-ray analyses revealed these compounds to have uncommon molecular architectures, with 3 being an unprecedented pyrophosphate-containing two-dimensional (2D) polymer. Compounds 1a/1b and 2 are discrete di- and tetra-nuclear complexes, respectively. The cationic {[Cu(phen)(H(2)O)](4)(HP(2)O(7))(2)}(2+) unit in 2 presents a unique quasi-flat structure, held together by solely in-plane pyrophosphate bridging modes (short O(eq)-P-O(eq) and long O(eq)-P-O-P-O(eq) pathways), a coordination arrangement also not previously reported. A different tetranuclear copper(II)-pyrophosphate arrangement is found in 3, with two classically bridged dimers (O(eq)-P-O(eq) pathway) joined together by auxiliary equatorial-axial μ-O pyrophosphate bridges. Here, the bidimensionality is reached through bridging phenam ligands, which provide further inter-"tetramer" metal-metal connections [(N,N')(eq)-(N')(ax) pathway], leading to the formation of an expanded covalent network based on the [Cu(2)(phenam)(2)(P(2)O(7))](2) moiety. Variable-temperature magnetic susceptibility measurements on polycrystalline samples of 2 and 3 revealed net antiferromagnetic coupling between metal centers with J(2a) = -7.9(2) cm(-1), J(2b) = -46.9(3) cm(-1), J(2c) = 0 cm(-1) in 2 (H = -J(2a)[S(Cu(1))·S(Cu(2)) + S(Cu(1a))·S(Cu(2a))] - J(2b)[S(Cu(1))·S(Cu(2a)) + S(Cu(1a))·S(Cu(2))] - J(2c)S(Cu(2))·S(Cu(2a))), and J(3a) = -87.9(2) cm(-1), J(3b) = -5(1) cm(-1) and J(3c) = +5(3) cm(-1) in 3 (H = -J(3a)[S(Cu(1))·S(Cu(2)) + S(Cu(1a))·S(Cu(2a))] - J(3b)[S(Cu(1))·S(Cu(2a)) + S(Cu(1a))·S(Cu(2))] - J(3c)S(Cu(2))·S(Cu(2a))). For 1a, a net ferromagnetic coupling is observed with J(1a) = +0.86(1) cm(-1) (H = -J S(A)·S(B) + S(A)·D· S(B) + βH (g(A)S(A) + g(B)S(B)). This is the first example of ferromagnetic coupling in pyrophosphate-complexes reported to date. A structure-function correlation study focusing on magnetic exchange across the observed diverse pyrophosphate-bridges is described with density functional theory (DFT) calculations included to support the stated observations.     

Fluorescence excitation and excited state intramolecular proton transfer of jet-cooled naphthol derivatives: part 2. 2-Hydroxy-1-naphthaldehyde

The S(1)← S(0) fluorescence excitation spectrum of jet-cooled 2-hydroxy-1-naphthaldehyde (2H1N) with origin at 26,668 cm(-1) has been measured. Nine totally symmetric modes and three non-totally symmetric modes have been assigned in the excitation spectrum. Ab initio calculations indicate that 2H1N undergoes a planarity change upon excitation, which may account for the unusual intensity of non totally symmetric vibrational modes in the excitation spectrum. A number of low intensity features were observed on the low energy side of the origin which have been assigned to the 2H1N dimer rather than different ground state confomers of 2H1N. The origin of the S(1)← S(0) electronic transition of the dimer lies at ~26,401 cm(-1); combinations of two low frequency intermolecular modes of the dimer (59 cm(-1) and 17 cm(-1)) were also observed. The occurrence of excited state intramolecular proton transfer (ESIPT) in 2H1N cannot be proven on the basis of this work. A comparison of the (photo)physical properties of 2H1N with 1-hydroxy-2-naphthaldehyde (1H2N) [A. McCarthy and A.A. Ruth, PCCP, 2011, 13, 7485-7499 (Part 1)], however, indicate the plausibility of an ESIPT process in 2H1N. The strength of the intramolecular hydrogen bond (IMHB) in 2H1N was computed as ~10.6 kcal/mol, a value comparable to the IMHB strength of 1H2N. The establishment of a lower limit on the state lifetimes of 2H1N, of ~1.8 ps, indicates that any proposed ESIPT reaction in 2H1N may not proceed barrierlessly. Above an excess energy of ~1000 cm(-1), the intensity of the fluorescence excitation spectrum reduces significantly, indicating the onset of a non-radiative decay mechanism.     

Internal rotation of methyl group in electronically excited o- and m-ethynyltoluene: new correlation between the Hammett substituent constant σm and rotational barrier change

We have determined the potential-energy function for the internal rotation of the methyl group for o- and m-ethynyltoluene in the electronic excited (S(1)) and ground (S(0)) states by measuring the fluorescence excitation and single-vibronic-level dispersed fluorescence spectra in a jet. The 0-0 bands were observed at 35?444 and 35?416 cm(-1), respectively. The methyl group in o-ethynyltoluene is shown to be a rigid rotor with a potential barrier to rotation of 190 ± 10 cm(-1) in both states. No change in the conformation occurred upon excitation. Barrier heights of m-ethynyltoluene in the S(0) and S(1) states are shown to be 19 ± 3 and 101 ± 1 cm(-1), respectively. A conformational change occurred with rotation by 60[ordinal indicator, masculine] upon excitation. The potential parameters were as follows: reduced rotational constant (B) of 5.323 cm(-1), centrifugal-distortion constant (D) of 6.481 × 10(-5) cm(-1), V(3) = 19 cm(-1), V(6) = -6 cm(-1), and V(9) = 0 cm(-1) in the S(0) state, and B = 5.015 cm(-1), D = 5.392 × 10(-5) cm(-1), V(3) = 101 cm(-1), V(6) = -22 cm(-1), and V(9) = -2 cm(-1) in the S(1) state. For m-methylstyrene, m-tolunitrile, and m-ethynyltoluene, which all have a multiple-bonded carbon in the substituent, we found a new correlation between the Hammett substituent constant σ(m) and the change in the barrier of the methyl group upon excitation.     

The 5f2-->5f16d1 absorption spectrum of Cs2GeF6:U4+ crystals: A quantum chemical and experimental study

Single crystals of U(4+)-doped Cs2GeF6 with 1% U4+ concentration have been obtained by the modified Bridgman-Stockbarger method in spite of the large difference in ionic radii between Ge4+ and U4+ in octahedral coordination. Their UV absorption spectrum has been recorded at 7 K, between 190 and 350 nm; it consists of a first broad and intense band peaking at about 38,000 cm(-1) followed by a number of broad bands of lower intensity from 39,000 to 45,000 cm(-1). None of the bands observed shows appreciable fine vibronic structure, so that the energies of experimental electronic origins cannot be deduced and the assignment of the experimental spectrum using empirical methods based on crystal field theory cannot be attempted. Alternatively, the profile of the absorption spectrum has been obtained theoretically using the U-F bond lengths and totally symmetric vibrational frequencies of the ground 5f2 - 1A(1g) and 5f16d(t(2g))1 - iT(1u) excited states, their energy differences, and their corresponding electric dipole transition moments calculated using the relativistic ab initio model potential embedded cluster method. The calculations suggest that the observed bands are associated with the lowest five 5f2 - 1A(1g)-->5f16d(t(2g))1 - iT(1u) (i = 1-5) dipole allowed electronic origins and their vibrational progressions. In particular, the first broad and intense band peaking at about 38,000 cm(-1) can be safely assigned to the 0-0 and 0-1 members of the a(1g) progression of the 5f2 - 1A(1g)-->5f16d(t(2g))1 - 1T(1u) electronic origin. The electronic structure of all the states with main configurational character 5f16d(t(2g))1 has been calculated as well. The results show that the lowest crystal level of this manifold is 5f16d(t(2g))1 - 1E(u) and lies about 6200 cm(-1) above the 5f2 level closest in energy, which amounts to some 11 vibrational quanta. This large energy gap could result in low nonradiative decay and efficient UV emission, which suggest the interest of investigating further this new material as a potential UV solid state laser.     

A combined zero electronic kinetic energy spectroscopy and ion-pair dissociation imaging study of the F2 + (X 2Pi g) structure

Rotationally resolved pulsed field ionization and zero electronic kinetic energy photoelectron spectra for the transition F(2) (+)(X (2)Pi(g))<--F(2)(X (1)Sigma(g) (+)) have been recorded using the extreme ultraviolet coherence radiation. The vibrational energy spacings, rotational constants, and spin orbit coupling constants for the first three vibrational states of F(2) (+)(X (2)Pi(g)) have been determined accurately. The first adiabatic ionization potential (IP) of F(2) is determined as IP(F(2))=126 585.7+/-0.5 cm(-1). To determine the threshold E(tipp) for ion-pair production of F(2), the images of F(-)((1)S(0)) in the velocity mapping conditions have also been recorded at the photon energy of 126 751 cm(-1). Taking the Stark effect into account, the E(tipp) is determined as E(tipp)(F(2))=126 045+/-8 cm(-1) (15.628+/-0.001 eV). By combing the IP(F(2)) and the E(tipp)(F(2)) determined in this work and together with the reported ionization potential and electronic affinity of the F atom, the bond dissociation energies of F(2) and F(2) (+) are determined as D(0)(F(2))=1.606+/-0.001 eV and D(0)(F(2) (+))=3.334+/-0.001 eV, respectively.     

Ab initio calculation of the NH(3sigma-)-NH(3sigma-) interaction potentials in the quintet, triplet, and singlet states

We present the ab initio potential-energy surfaces of the NH-NH complex that correlate with two NH molecules in their 3sigma- electronic ground state. Three distinct potential-energy surfaces, split by exchange interactions, correspond to the coupling of the S(A) = 1 and S(B) = 1 electronic spins of the monomers to dimer states with S = 0, 1, and 2. Exploratory calculations on the quintet (S = 2), triplet (S = 1), and singlet (S = 0) states and their exchange splittings were performed with the valence bond self-consistent-field method that explicitly accounts for the nonorthogonality of the orbitals on different monomers. The potential surface of the quintet state, which can be described by a single Slater determinant reference function, was calculated at the coupled cluster level with single and double excitations and noniterative treatment of the triples. The triplet and singlet states require multiconfiguration reference wave functions and the exchange splittings between the three potential surfaces were calculated with the complete active space self-consistent-field method supplemented with perturbative configuration interaction calculations of second and third orders. Full potential-energy surfaces were computed as a function of the four intermolecular Jacobi coordinates, with an aug-cc-pVTZ basis on the N and H atoms and bond functions at the midpoint of the intermolecular vector R. An analytical representation of these potentials was given by expanding their dependence on the molecular orientations in coupled spherical harmonics, and representing the dependence of the expansion coefficients on the intermolecular distance R by the reproducing kernel Hilbert space method. The quintet surface has a van der Waals minimum of depth D(e) = 675 cm(-1) at R(e) = 6.6a0 for a linear geometry with the two NH electric dipoles aligned. The singlet and triplet surfaces show similar, slightly deeper, van der Waals wells, but when R is decreased the weakly bound NH dimer with S = 0 and S = 1 converts into the chemically bound N2H2 diimide (also called diazene) molecule with only a small energy barrier to overcome.     

Spectral tuning by switching C-H[...]O hydrogen bonds: rotation-induced spectral shifts of 7-hydroxyquinoline HCOOH isomers

Spectral tuning effects on visible chromophores by hydrogen bonds are central to the chemistry of vision and of photosynthesis. A model for large spectral tuning effects by hydrogen bond switching is provided by the 7-hydroxyquinoline x HCOOH complex, which forms two isomers, CTN1 and CTN2, both with an HCOOH[...]N hydrogen bond but with different (quinoline)C-H[...]O=C hydrogen bonds. A 180 degrees rotation of the HCOOH moiety around the O-H[...]N hydrogen bond exchanges the C-H[...]O hydrogen bonds, rotates the dipole moment of HCOOH, and leads to an approximately 850 cm(-1) shift of the electronic spectrum. Mass-selected S1<--S0 resonant two-photon ionization, UV-UV holeburning, S1-->S0 fluorescence spectra, and photoionization efficiency curves of the two 7-hydroxyquinoline x HCOOH isomers were measured in supersonic expansions. Comparison to ab initio calculations allow us to determine the H-bond connectivity and structure of the two isomers and to assign their inter- and intramolecular vibrations. The Franck-Condon factors of the intermolecular shear vibration chi in the S1<--S0 spectra indicate that the weak C-H[...]O hydrogen bond contracts markedly in the CTN1 isomer but expands in the CTN2 isomer. These changes of H-bond lengths agree with the spectral shifts. In contrast, the strong O-H[...]N hydrogen bond undergoes little change upon S1[...]S0 excitation.     

New view of the ICN A continuum using photoelectron spectroscopy of ICN-

Negative-ion photoelectron spectroscopy of ICN(-) (X??(2)Σ(+)) reveals transitions to the ground electronic state (X??(1)Σ(+)) of ICN as well as the first five excited states ((3)Π(2), (3)Π(1), Π(0(-) ) (3), Π(0(+) ) (3), and (1)Π(1)) that make up the ICN A continuum. By starting from the equilibrium geometry of the anion, photoelectron spectroscopy characterizes the electronic structure of ICN at an elongated I-C bond length of 2.65 A?. Because of this bond elongation, the lowest three excited states of ICN ((3)Π(2), (3)Π(1), and Π(0(-) ) (3)) are resolved for the first time in the photoelectron spectrum. In addition, the spectrum has a structured peak that arises from the frequently studied conical intersection between the Π(0(+) ) (3) and (1)Π(1) states. The assignment of the spectrum is aided by MR-SO-CISD calculations of the potential energy surfaces for the anion and neutral ICN electronic states, along with calculations of the vibrational levels supported by these states. Through thermochemical cycles involving spectrally narrow transitions to the excited states of ICN, we determine the electron affinity, EA(ICN), to be 1.34(5) (+0.04∕-0.02) eV and the anion dissociation energy, D(0)(X??(2)Σ(+) I-CN(-)), to be 0.83 (+0.04/-0.02) eV.     

铁原子与氮分子间的相互作用——单侧双配位构型

用密度泛函理论的B3LYP/6-311+G(d)方法对单侧双配位FeN2体系(简记为S-FeN2)不同自旋多重度的稳定态、范德华力作用态和过渡态的多个电子态的几何结构、电子结构、能量和振动频率进行了计算比较研究. 结果表明, S-FeN2体系三种自旋态间, Fe—N 距离R1和N—N 距离R2值均比较接近; 能量最低的是15B2态, 相近态有15B1、13B1和13B2, 彼此能差约25 kJ·mol-1. 三重态电子结构复杂, 单重态能量普遍偏高; 基组态Fe原子与N2间存在强的σ-π电子对排斥而无有效轨道重叠和电子转移, 其它组态4s13d7、4s13d64p1和3d74p1, Fe 和N2间发生σ(sd)-π和π-π*轨道重叠作用, 有少量电子转移, 体系呈现一定的离子性特征, 活化N2键长基本不超过120 pm. Fe 原子的电子单或双重被激发到由N2反键轨道为主要成分的分子轨道上时, 能使N2活化到单键程度甚至解离.     

Anharmonic vibrational frequencies and vibrationally averaged structures and nuclear magnetic resonance parameters of FHF-

The anharmonic vibrational frequencies of FHF(-) were computed by the vibrational self-consistent-field, configuration-interaction, and second-order perturbation methods with a multiresolution composite potential energy surface generated by the electronic coupled-cluster method with various basis sets. Anharmonic vibrational averaging was performed for the bond length and nuclear magnetic resonance indirect spin-spin coupling constants, where the latter computed by the equation-of-motion coupled-cluster method. The calculations placed the vibrational frequencies at 580 (nu(1)), 1292 (nu(2)), 1313 (nu(3)), 1837 (nu(1) + nu(3)), and 1864 cm(-1) (nu(1) + nu(2)), the zero-point H-F bond length (r(0)) at 1.1539 A, the zero-point one-bond spin-spin coupling constant [(1)J(0)(HF)] at 124 Hz, and the bond dissociation energy (D(0)) at 43.3 kcal/mol. They agreed excellently with the corresponding experimental values: nu(1) = 583 cm(-1), nu(2) = 1286 cm(-1), nu(3) = 1331 cm(-1), nu(1) + nu(3) = 1849 cm(-1), nu(1) + nu(2) = 1858 cm(-1), r(0) = 1.1522 A, (1)J(0)(HF) = 124+/-3 Hz, and D(0) = 44.4+/-1.6 kcal/mol. The vibrationally averaged bond lengths matched closely the experimental values of five excited vibrational states, furnishing a highly dependable basis for correct band assignments. An adiabatic separation of high- (nu(3)) and low-frequency (nu(1)) stretching modes was examined and found to explain semiquantitatively the appearance of a nu(1) progression on nu(3). Our calculations predicted a value of 186 Hz for experimentally inaccessible (2)J(0)(FF).