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We present a versatile one-pot synthesis method for creating surface-anchored orthogonal gradient networks using a small bi-functional gelator, 4-azidosulfonylphenethyltrimethoxysilane (4-ASPTMS). The sulfonyl azide (SAz) group of 4-ASPTMS is UV (≤254 nm) and thermally active (≥100 °C) and, thus, enables us to vary the cross-link density in orthogonal directions by controlling the activation of SAz groups via UV and temperature means. We deposit a thin layer (∼200 nm) of a mixture comprising ∼90% precursor polymer and ∼10% 4-ASPTMS in a silicon wafer. Upon UV irradiation or annealing the layers, SAz releases nitrogen by forming singlet and triplet nitrenes that concurrently react with any C–H bond in the vicinity leading to sulfonamide cross-links. Condensation among trimethoxy groups in the bulk connects 4-ASPTMS units and completes the cross-linking. Simultaneously, 4-ASPTMS near the substrate reacts with surface-bound –OH motifs that anchor the cross-linked polymer chains to the substrate. We demonstrate the generation of orthogonal gradient network coatings exhibiting cross-link density (or stiffness) gradients in orthogonal directions using such a simple process. [ABSTRACT FROM AUTHOR]

First W-band 94 GHz EPR spectra of randomly oriented triplet, quintet, and septet nitrenes formed during the photolysis of 1,3,5-triazido-2,4,6-tribromobenzene in cryogenic matrices are reported. In comparison with conventional X-band 9 GHz electron paramagnetic resonance (EPR) spectroscopy, W-band EPR spectroscopy allows the detection and complete spectroscopic characterization of all paramagnetic species formed at different stages of the photolysis of aromatic polyazides. This type of spectroscopy is of paramount importance for experimental determination of the sign of the zero-field splitting (ZFS) parameters of high-spin molecules with large spin-orbit contribution to the ZFS, caused by the effect of heavy atoms. The study shows that triplet 1,3-diazido-2,4,6- tribromo-5-nitrenobenzene (T1) has DT = 1.369 cm-1,ET = 0.093 cm-1, and g = 2.0033, quintet 1- azido-2,4,6-tribromo-3,5-dinitrenobenzene (Q1) shows DQ = -0.306 cm-1,EQ = 0.0137 cm-1, and g = 2.0070, and septet 2,4,6-tribromo-1,3,5-trinitrenobenzene (S1) has DS = -0.203 cm-1, ES = 0, and g = 2.0073. The experimental ZFS parameters agree well with the results of density functional theory calculations at the PBE/Ahlrichs-DZ level of theory, showing that such calculations adequately describe the magnetic properties of bromine-containing high-spin nitrenes. Both experimental and theoretical data indicate that, in contrast to all known to date quintet dinitrenes, dinitrene Q1 has the negative sign of magnetic anisotropy due to the "heavy atom effect." This dinitrene along with septet trinitrene S1 possess the largest negative value of D among all known quintet and septet organic polyradicals. [ABSTRACT FROM AUTHOR]

We investigated the optical properties of 4,4′-stilbenedinitrene at low temperature and in high magnetic fields and compared the results with complementary first principles calculations. Both physical tuning parameters allow us to manipulate the singlet-triplet equilibrium, and by doing so, control the optical contrast (which is on the order of -2.5 × 102 cm-1 at 555 nm and 35 T). Moreover, analysis of the magneto-optical response using a combined population and Beer's law framework reveals the singlet-triplet spin gap and identifies particular features in the absorption difference spectrum as deriving from singlet or triplet state excitations. These findings deepen our understanding of coupling in open shell molecules and show how chemical structure modification can modulate charge-spin interactions in organic biradicals. [ABSTRACT FROM AUTHOR]

High-spin organic molecules with dominant spin-orbit contribution to magnetic anisotropy are reported. Quintet 4-azido-3,5-dibromopyridyl-2,6-dinitrene (Q-1), quintet 2-azido-3,5-dibromopyridyl-4,6-dinitrene (Q-2), and septet 3,5-dibromopyridyl-2,4,6-trinitrene (S-1) were generated in solid argon matrices by ultraviolet irradiation of 2,4,6-triazido-3,5-dibromopyridine. The zero-field splitting (ZFS) parameters, derived from electron spin resonance spectra, show unprecedentedly large magnitudes of the parameters D: |DQ1| = 0.289, |DQ2| = 0.373, and |DS1| = 0.297 cm-1. The experimental ZFS parameters were successfully reproduced by density functional theory calculations, confirming that magnetic anisotropy of high-spin organic molecules can considerably be enhanced by the 'heavy atom effect.' In bromine-containing high-spin nitrenes, the spin-orbit term is dominant and governs both the magnitude and the sign of magnetic anisotropy. The largest negative value of D among septet trinitrenes is predicted for 1,3,5-trinitrenobenzene bearing three heavy atoms (Br) in positions 2, 4, and 6 of the benzene ring. [ABSTRACT FROM AUTHOR]

We investigated the tunability of the singlet-triplet equilibrium population in the organic biradical 1,4-phenylenedinitrene via magneto-optical spectroscopy. A rich magnetochromic response occurs because applied field increases the concentration of the triplet state species, which has a unique optical signature by comparison with the singlet biradical and the precursor molecule. A Curie-like analysis of the magneto-optical properties allows us to extract the spin gap, which is smaller than previously supposed. These measurements establish the value of local-probe photophysical techniques for magnetic property determination in open-shell systems such as biradicals where a traditional electron paramagnetic resonance Curie law analysis has intrinsic limitations. [ABSTRACT FROM AUTHOR]

The kinetics of azobenzene nitrene oxidation by molecular oxygen dissolved in a matrix was studied in glassy propylene carbonate. The reaction was carried out in excess oxygen within its concentration range 0.008 to 0.048 M. The oxidation kinetics, controlled by oxygen diffusion, is not described by the exponential function. A specific reaction rate decreases in the course of the process. The higher the oxygen concentration in the matrix, the sharper is the decrease in the reaction rate. It is shown that at all concentrations, the oxidation kinetics is described in the framework of the model of heterogeneous matrix with a characteristic size of heterogeneities of 1.5 nm and a dispersion of the activation energy of oxygen molecule jump of 4 kJ/mol. The reaction radius is about 0.5 nm. [ABSTRACT FROM AUTHOR]

This work presents a detailed evaluation of the performance of density functional theory (DFT) for the prediction of zero-field splittings (ZFSs) in high-spin nitrenes. A number of well experimentally characterized triplet mononitrenes, quartet nitrenoradicals, quintet dinitrenes, and septet trinitrenes have been considered. Several DFT-based approaches for the prediction of ZFSs have been compared. It is shown that the unrestricted Kohn–Sham and the Pederson–Khanna approaches are the most successful for the estimation of the direct spin-spin (SS) interaction and the spin-orbit coupling (SOC) parts, respectively, to the final ZFS parameters. The most accurate theoretical predictions (within 10%) are achieved by using the PBE density functional in combination with the DZ, EPR-II, and TZV basis sets. For high-spin nitrenes constituted from light atoms, the contribution of the SOC part to ZFS parameters is quite small (7%–12%). By contrast, for chlorine-substituted septet trinitrenes, the contribution of the SOC part is small only to D value but, in the case of E value, it is as large as the SS part and has opposite sign. Due to this partial cancellation of two different contributions, SS and SOC, the resulting values of E in heavy molecules are almost two times smaller than those predicted by analysis of the widely used semiempirical one-center spin-spin interaction model. The decomposition of DSS into n-center (n=1–4) interactions shows that the major contribution to DSS results from the one-center spin-spin interactions. This fact indicates that the semiempirical SS interaction model accurately predicts the ZFS parameters for all types of high-spin nitrenes with total spin S=2 and 3, if their molecules are constructed from the first-row atoms. [ABSTRACT FROM AUTHOR]

Analytic gradients for the state-specific multireference coupled-cluster method suggested by Mahapatra et al. [Mol. Phys. 94, 157 (1998)] (Mk-MRCC) are reported within the singles and doubles approximation using two-configurational self-consistent field (TCSCF) orbitals. The present implementation extends our previous work on Mk-MRCC gradients [E. Prochnow et al., J. Chem. Phys. 131, 064109 (2009)] which is based on restricted Hartree–Fock orbitals and consequently the main focus of the present paper is on the treatment of orbital relaxation at the TCSCF level using coupled-perturbed TCSCF theory. Geometry optimizations on m-arynes and nitrenes are presented to illustrate the influence of the orbitals on the computed equilibrium structures. The results are compared to those obtained at the single-reference coupled-cluster singles and doubles and at the Mk-MRCC singles and doubles level of theory when using restricted Hartree–Fock orbitals. [ABSTRACT FROM AUTHOR]

Infrared predissociation spectroscopy is carried out for the structure investigation of unprotonated cluster cations of protic molecules such as ammonia and methanol, which are generated through vacuum-ultraviolet one-photon ionization of their jet-cooled neutral clusters. The observed spectral features show that the cluster cations have the proton-transferred type structures, where a pair of a protonated cation and a neutral radical, NH4+...NH2 or CH3OH2+...OCH3, is formed. Theoretical calculations at the MP2 and B3LYP levels support the formation of the proton-transferred type structures for the cluster cations, and indicate that they are formed by proton-transfer following the photoionization of the neutral clusters. [ABSTRACT FROM AUTHOR]

An extension of Cohen’s nuclear Fukui function is presented in the spin-polarized framework of density-functional theory (SP-DFT). The resulting new nuclear Fukui function indices [uppercase_phi_synonym]Nα and [uppercase_phi_synonym]Sα are intended to be the natural descriptors for the responses of the nuclei to changes involving charge transfer at constant multiplicity and also the spin polarization at constant number of electrons. These generalized quantities allow us to gain new insights within a perturbative scheme based on DFT. Calculations of the electronic and nuclear SP-DFT quantities are presented within a Kohn-Sham framework of chemical reactivity for a sample of molecules, including H2O, H2CO, and some simple nitrenes (NX) and phosphinidenes (PX), with X=H, Li, F, Cl, OH, SH, NH2, and PH2. Results have been interpreted in terms of chemical bonding in the context of Berlin’s theorem, which provides a separation of the molecular space into binding and antibinding regions. [ABSTRACT FROM AUTHOR]

Studies the collision-induced intersystem crossing from imidogen (NH) radicals. Optical emission observed issuing from the collision zone; Two components revealed by the spectral analysis; Calculated branching ratio of the fast and slow components.

The vibrationally resolved laser-induced fluorescence spectra of the A 3EX 3A2 system of CH3N radical were obtained experimentally. The dispersed spectra at different vibrationally excited states confirm that the system studied belongs to the CH3N radical. From the vibrational progressions in the excitation spectrum, following vibrational frequencies of the A 3E state were determined: ν1′=2943 cm-1, ν2′=1239 cm-1, ν3′=759 cm-1, and ν5′=1500 cm-1. In addition, the lifetimes of the A 3E state at four vibrational states (ν3′=0, 3, 4, and 6) were measured, which are, respectively 292.8±1.2, 203.4±1.2, 188.2±0.5, and 157.2±0.5 ns at a total pressure of 1.7 Torr. Our experiments show that the A 3E state is bound at least up to 4800 cm-1 and is relatively free from photochemistry. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reaction of NH(a 1Δ) with methanol has been studied by a flash photolysis of HNCO at 193 nm at room temperature (298±2 K). Time dependence and relative concentrations for NH, NH2, NHD, CH3O, H, and D are monitored by a laser induced fluorescence (LIF) technique. From the pseudo-first-order analysis of NH(a 1Δ) decay rate, the rate constants for the reactions NH(a 1Δ)+CH3OH→ products (1), and NH(a 1Δ)+CH3OD→products (2) have been determined to be k1=(1.37±0.10)×10-10 cm3 molecule-1 s-1 in the pressure range of 3–20 Torr Ar or 5 Torr He, and k2=(1.33±0.05)×10-10 cm3 molecule-1 s-1 at the pressure of 5 Torr He, respectively. By comparing the LIF intensity of NH(X 3Σ-) in reaction (1) with that in NH(a 1Δ)+Xe reaction system, the contribution of physical quenching in reaction (1) has been evaluated to be less than 2.4%. Also by comparing the LIF intensities of NHD and NH2 in reaction (2) with those in NH(a 1Δ)+D2/H2 reaction systems, the branching ratio between the pathways producing NHD (2b) and NH2 (2c) has been determined to be k(2b)/k(2c)=23±9. It is elucidated that the insertion of NH(a 1Δ) into O–D bond of CH3OD is completely predominant over C–H bond; i.e., the O–D bond is about 69 times more reactive toward insertion of NH(a 1Δ) than a single C–H bond. [ABSTRACT FROM AUTHOR]

The optical emission spectrum of the methylnitrene radical has been observed at a resolution of 0.07 cm-1. Transitions to subbands with K up to 3 in the X 3A2 ground state were observed and a full rotational analysis carried out. Comparison of the structural information with the highest level theoretical calculations shows agreement to within the estimated error. [ABSTRACT FROM AUTHOR]

Two-color mass resolved excitation spectra are obtained for the benzyl and ‘‘phenylnitrene’’ radicals. The spectrum previously assigned to the benzyl radical is generated by a species with mass 91 amu. This finding is consistent with the proposed C6H5ĊH2 benzyl radical structure. The origin of this 1 2A2←1 2A1 transition lies at 21 997 cm-1. The ionization energy for the benzyl radical is ∼7.236 eV. The phenylnitrene radical spectrum is associated with a species of 90 amu. The phenylnitrene designation is not appropriate for this mass number: the suggested cyanocyclopentadienyl radical species (C5H4CN) is consistent with this mass. This radical has an ionization threshold of ∼9.05 eV. Lifetimes are reported for a number of the vibronic states of both radicals. [ABSTRACT FROM AUTHOR]

Reactions of imidogen, NH, in the electronically excited a 1Δ and ground X 3Σ- states with NO have been studied at 300±2 K and at 20 Torr (Ar). NH(a 1Δ) and NH(X 3Σ-) were produced in the 193 nm photolysis of HNCO and CHBr3/NO, respectively, and detected by the laser-induced fluorescence (LIF) technique. Rate constants have been measured under pseudo-first-order conditions by scanning the time delay between the photolysis and probe laser pulses. Overall rate constants were determined: [4.8±0.1(2σ)]×10-11 cm3 molecule-1 s-1 for NH(a 1Δ)+NO; [4.7±0.6(2σ)]×10-11 cm3 molecule-1 s-1 for NH(X 3Σ-)+NO. The ratio of electronic quenching, NH(a 1Δ)+NO→NH(X 3Σ-) +NO, to the overall reaction has been obtained to be 0.53±0.1. An interesting specificity for the products from different spin states of the reactants has been found from observation of OH concentration profiles. NH(X 3Σ-)+NO produces OH+N2 exclusively, while NH(a 1Δ)+NO is over five times less effective in producing OH+N2. Profiles of H atoms detected by the vuv LIF technique indicated that H atoms are directly produced in the reaction NH(a 1Δ)+NO but not in the reaction NH(X 3Σ-)+NO. [ABSTRACT FROM AUTHOR]

The recently formulated multiconfiguration-based unitary coupled electron pair approximation (UCEPA) is compared with multireference configuration interaction (MR-CISD) calculations, including all single and double excitations, for the molecules in this study. The electronic states of the molecules in this study are not only of experimental interest, but represent a challenge to any formalism to accurately predict the energy separations of the low-lying electronic states. The equilibrium geometries and fundamental vibrational frequencies of the three lowest electronic states (i.e., 1A1, 3A‘, and 1A‘) of aminonitrene H2N2, and phosphinonitrene, H2PN, have been determined using a split-valence basis with polarization functions on the heavy atoms and a small complete active space self-consistent-field (CASSCF) description of the active space. Both MR-CISD and UCEPA calculations have been performed at the equilibrium structures using larger basis sets to accurately determine the relative energetics of the electronic states. The equilibrium geometries and vibrational frequencies of the two lowest electronic states (i.e., 1A’ and 3A‘) of phosphinocarbene, H2PCH, have been determined using a larger than double zeta basis set, augmented with polarization and diffuse functions, and a CASSCF description of the active space. Both MR-CISD and UCEPA calculations were performed on the equilibrium structures and predict that the singlet lies between 10.4 and 11.8 kcal/mol lower in energy than the triplet. The use of a generalized valence bond (GVB) reference function within UCEPA is introduced and is shown to be a useful approximation. [ABSTRACT FROM AUTHOR]

The methylnitrene radical CH3N has been produced by the interaction of metastable molecular nitrogen and methylazide, and has been trapped in solid nitrogen at 10 K. The IR and UV absorption spectra are consistent with gas phase emission results, and also show transitions to several additional vibrational levels in the A 3E excited electronic state. The UV band system consists chiefly of a progression in the C–N stretching mode, possibly complicated by additional unresolved vibrational structure. These observations indicate that the absence of extensive hot band structure in the gas phase work is not due to rapid dissociation or isomerization from the upper state, which is bound by at least 3500 cm-1. The ESR signals previously attributed to CH3N cannot be produced under the same experimental conditions, but require additional photolysis. This observation suggests that a reexamination of previous ESR data is warranted. [ABSTRACT FROM AUTHOR]

New observations have enabled the identification of another excited state vibration in the A 3E –X 3A2 ultraviolet absorption spectrum of matrix-isolated methylnitrene. Two progressions in the C–N stretching frequency ν3 were detected in both CH3N and CD3N, with average intervals of 710 and 695 cm-1, respectively. For each isotopomer, corresponding lines in the pair of progressions are offset by frequencies appropriate for a methyl vibration. This provides the first clear evidence for excitation of the CH3 symmetric deformation (umbrella mode) ν2 in the upper state of the transition, with vibrational frequencies of 1166 cm-1 for CH3N and 1044 cm-1 for CD3N. These observations confirm earlier speculation on the absorption spectrum of matrix-isolated CH3N, and correlate with unidentified lines in the gas phase emission spectrum. The use of a new precursor, methyl isocyanate, in the generation of this radical is also of interest. [ABSTRACT FROM AUTHOR]