Spectrochemical, ab initio and density functional studies on the conversion of 2-hydroxybenzonitrile (o-cyanophenol) into the oxyanion

The spectral and structural changes caused by the conversion of 2-hydroxybenzonitrile (o-cyanophenol) into the corresponding oxyanion have been followed by IR spectra, ab initio and density functional force field calculations. In agreement between theory and experiment, the conversion is accompanied by a 29 cm(-1) frequency decrease of the cyano stretching band, 2.7-fold increase in its integrated intensity, 5.8-fold (total value) intensification of the aromatic skeletal bands of Wilson's 8 and 19 types, and other essential spectral changes. According to the calculations, the strongest structural changes are the shortening of the Ph-O bond with 0.10 A, lengthenings of the adjacent CC bonds in the phenylene ring with 0.06 A and bond angle variations near the oxyanionic center. All these changes are connected with the formation of a quasi-ortho-quinonoidal structure of the o-phenylene ring in the oxyanion. According to the electronic density analysis, 0.41 e(-) (Mulliken) or 0.56 e(-) (natural bond orbital, NBO) of the anionic charge remain localized at the oxyanionic center. Conformations and hydrogen bonds have also been discussed on the basis of experimental and theoretical data.     

IR spectral and structural changes caused by the conversion of 3-hydroxybenzaldehyde into the oxyanion

The spectral and structural changes, caused by the conversion of the 3-hydroxybenzaldehyde molecule into the corresponding oxyanion have been studied by IR spectra, and MP2 and DFT force field calculations. The conversion causes a 13 cm(-1) decrease in the frequency of the carbonyl stretching band nu(Cz=O), a 1.3-fold increase in its integrated intensity, strong intensity increases (2.1-5.3-fold) of the aromatic skeletal nu8 and nu19 as well as formyl nu(CH) bands. According to the calculations the oxyanionic charge is delocalized over aldehyde group (0.37 e-), phenylene ring (0.12 e-) and oxyanionic center (0.52 e-). The conversion into the oxyanion leads to geometry changes in the whole species, but it remains planar.     

IR spectral and structural changes caused by the conversion of 3-methoxy-4-hydroxybenzaldehyde (vanillin) into the oxyanion

The spectral and structural changes, caused by the conversion of the vanillin molecule into the corresponding oxyanion have been studied by IR spectra and normal coordinate calculations within the Onsager self-consistent reaction field (SCRF) model, using a density functional theory (DFT) method at the Becke3LYP/6-31+G** level. Structures of all conformational isomers of vanillin and of its anion have been located, as well as their total and relative energies have been determined. The conversion leads to geometry changes in the whole species, due to the strong O-/acceptor polar resonance through the para phenylene ring. The conversion causes a 41 cm(-1) decrease in the frequency of the carbonyl stretching band nu(C=O), strong intensity increases (1.6 - 7.2-fold) of the aromatic skeletal nu8 and nu19 as well as formyl nu(CH) bands. According to the calculations the oxyanionic charge is delocalized over aldehyde group (0.25 e-), phenylene ring (0.13 e-), methoxy group (0.07 e-) and oxyanyonic center (0.55 e-).     

Selective binding of the cyano group in acrylonitrile adsorption on Si(100)-2 x 1

The covalent binding of acrylonitrile (CH(2)=CH-C triple bond N) and the formation of a C=C-C=N structure on Si(100) have been investigated using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and density functional theory (DFT) calculations. For chemisorbed acrylonitrile, the absence of nu(C triple bond N) at 2245 cm(-1) and the appearance of nu(C=N) at 1669 cm(-1) demonstrate that the cyano group directly participates in the interaction with Si(100), which is further supported by XPS and UPS observations. Our experimental results and DFT calculations unambiguously demonstrate a [2 + 2] cycloaddition mechanism for acrylonitrile chemisorption on Si(100) through the binding of C triple bond N to Si dimers. The resulting chemisorbed monolayer with a C=C-C=N skeleton can serve as a precursor for further chemical syntheses of multilayer organic thin films in a vacuum and surface functionalization for in situ device fabrication.     

Synthesis of [F3S(triple bond)NXeF][AsF6] and structural study by multi-NMR and Raman spectroscopy, electronic structure calculations, and X-ray crystallography

The salt, [F3S(triple bond)NXeF][AsF6], has been synthesized by the reaction of [XeF][AsF6] with liquid N(triple bond)SF3 at -20 degrees C. The Xe-N bonded cation provides a rare example of xenon bound to an inorganic nitrogen base in which nitrogen is formally sp-hybridized. The F3S(triple bond)NXeF+ cation was characterized by Raman spectroscopy at -150 degrees C and by 129Xe, 19F, and 14N NMR spectroscopy in HF solution at -20 degrees C and in BrF5 solution at -60 degrees C. Colorless [F3S(triple bond)NXeF][AsF6] was crystallized from HF solvent at -45 degrees C, and its low-temperature X-ray crystal structure was determined. The Xe-N bond is among the longest Xe-N bonds known (2.236(4) A), whereas the Xe-F bond length (1.938(3) A) is significantly shorter than that of XeF2 but longer than in XeF+ salts. The Xe-F and Xe-N bond lengths are similar to those of HC(triple bond)NXeF+, placing it among the most ionic Xe-N bonds known. The nonlinear Xe-N-S angle (142.6(3)o) contrasts with the linear angle predicted by electronic structure calculations and is attributed to close N...F contacts within the crystallographic unit cell. Electronic structure calculations at the MP2 and DFT levels of theory were used to calculate the gas-phase geometries, charges, bond orders, and valencies of F3S(triple bond)NXeF+ and to assign vibrational frequencies. The calculated small energy difference (7.9 kJ mol-1) between bent and linear Xe-N-S angles also indicates that the bent geometry is likely the result of crystal packing. The structural studies, natural bond orbital analyses, and calculated gas-phase dissociation enthalpies reveal that F3S(triple bond)NXeF+ is among the weakest donor-acceptor adducts of XeF+ with an Xe-N donor-acceptor interaction that is very similar to that of HC(triple bond)NXeF+, but considerably stronger than that of F3S(triple bond)NAsF5. Despite the low dissociation enthalpy of the donor-acceptor bond in F3S(triple bond)NXeF+, 129Xe, 19F, and 14N NMR studies reveal that the F3S(triple bond)NXeF+ cation is nonlabile at low temperatures in HF and BrF5 solvents.     

Vibrational spectroscopic properties of hydrogen bonded acetonitrile studied by DFT

Vibrational properties (band position, Infrared and Raman intensities) of the acetonitrile C[triple bond]N stretching mode were studied in 27 gas-phase medium intensity (length range: = 1.71-2.05 angstroms; -deltaE range = 13-48 kJ/mol) hydrogen-bonded 1:1 complexes of CH3CN with organic and inorganic acids using density functional theory (DFT) calculations [B3LYP-6-31++G(2d,2p)]. Furthermore, general characteristics of the hydrogen bonds and vibrational changes in the OH stretching band of the acids were also considered. Experimentally observed blue-shifts of the C[triple bond]N stretching band promoted by the hydrogen bonding, which shortens the triple bond length, are very well reproduced and quantitatively depend on the hydrogen bond length. Both predicted enhancement of the infrared and Raman nu(C[triple bond]N) band intensities are in good agreement with the experimental results. Infrared band intensity increase is a direct function of the hydrogen bond energy. However, the predicted increase in the Raman band intensity increase is a more complex function, depending simultaneously on the characteristics of both the hydrogen bond (C[triple bond]N bond length) and the H-donating acid polarizability. Accounting for these two parameters, the calculated nu(C[triple bond]N) Raman intensities of the complexes are explained with a mean error of +/- 2.4%.     

Ab initio HF, density functional and experimental studies on the IR spectra and structure of 1,2-benzisothiazol-3-(2H)-thione-1,1-dioxide (thiosacchanin) and its nitranion

The spectral and structural changes taking place in the course of the conversion of 1,2-benzisothiazol-3-(2H)-thione-1,1-dioxide (thiosaccharin) into a nitranion have been studied on the basis of both IR spectra and ab initio HF 6-31G(d) and BLYP 6-31G(d,p) force field calculations. The conversion causes nu(as)SO2 and nu(s)SO2 frequency decreases of 47 and 13 cm(-1), respectively, and other spectral changes. The nuC=S coordinate is strongly delocalized. The ab initio geometries of the isolated molecule and nitranion agree well with the single-crystal X-ray ones, determined for thiosaccharin and its sodium (potassium) monohydrate salts, respectively. The nitranionic charge is delocalized almost uniformly within the thiocarbonyl (0.29 e-), sulfonyl (0.24 e-), and phenylene (0.24 e-) groups, and the nitranionic center (0.23 e-).     

Accurate determination of the structure of cyclooctatetraene by femtosecond rotational coherence spectroscopy and ab initio calculations

We combine femtosecond time-resolved rotational coherence spectroscopy with high-level ab initio theory to obtain accurate structural information for the nonpolar antiaromatic molecule 1,3,5,7-cyclooctatetraene (C8H8, COT) and its perdeuterated isotopomer COT-d8 (C8D8). We measure the rotational B0 and centrifugal distortion constants D(J), D(JK) of the v = 0 states of COT and COT-d8 to high accuracy, e.g. B0 (COT) = 2710.329(56) MHz, as well as B(v) for the v = 1 states nu6, nu11, nu17, nu22, and nu41/nu42 of COT. The experimental rotational constants are compared to those obtained from calculations at the coupled-cluster with single, double, and perturbative triples [CCSD(T)] level. The latter also take into account vibrational averaging effects of the ground and vibrationally excited states. Combining the experimental and calculated rotational constants with the calculated equilibrium bond lengths and angles allows us to determine accurate equilibrium structure parameters, e.g., r(e) (C-C) = 147.0 +/- 0.05 pm, r(e) (C=C) = 133.7 +/- 0.1 pm, and r(e) (C-H) = 107.9 +/- 0.1 pm. The equilibrium C-C and C=C bond lengths of COT are compared to those of 1,3-butadiene. The expected effect of decreased pi-electron delocalization due to the twisting of adjacent C=C double bonds in COT relative to butadiene is observed for the C-C bonds but not for the C=C bonds.     

Dry synthesis of triple cumulative double bonds (C=C=C=N) on Si(111)-7 x 7 surfaces

The interactions of cyanoacetylene and diacetylene with a Si(111)-7 x 7 surface have been studied as model systems to mechanistically understand the chemical binding of unsaturated organic molecules to diradical-like silicon dangling bonds. Vibrational studies show that cyanoacetylene mainly binds to the surface through a diradical reaction involving both cyano and C[triple bond]C groups with an adjacent adatom-rest atom pair at 110 K, resulting in an intermediate containing triple cumulative double bonds (C=C=C=N). On the other hand, diacetylene was shown to the covalently attached to Si(111)-7 x 7 only through one of its C[triple bond]C groups, forming an enynic-like structure with a C=C-C[triple bond]C skeleton. These chemisorbed species containing triple cumulative double bonds (C=C=C=N) and C=C-C[triple bond]C may be employed as precursors (or templates) for further construction of bilayer organic films on the semiconductor surfaces.     

Molecular structure of the octatetranyl anion, C8H-: a computational study

The equilibrium molecular structure of the octatetranyl anion, C8H(-), which has been recently detected in two astronomical environments, is investigated with the aid of both ab initio post-Hartree-Fock and density functional theory (DFT) calculations. The model chemistry adopted in this study was selected after a series of benchmark calculations performed on molecular acetylene for which accurate gas-phase structural data are available. Geometry optimizations performed at the CCSD/6-311+G(2d,p), QCISD/6-311+G(2d,p), and MP4(SDQ)/6-311+G(2d,p) levels of theory yield for C8H(-) an interesting polyyne-type structure that defies the chemical formula displaying a simple alternation of triple and single carbon-carbon bonds, [:C[triple bond]C-C[triple bond]C-C[triple bond]C-C[triple bond]CH](1-). In the optimized geometry of C8H(-), as one proceeds from the naked carbon atom on one side of the chain to the CH unit on the opposite side of the chain, the short (formally triple) carbon-carbon bonds decrease in length from 1.255 to 1.213 A whereas the long (formally single) carbon-carbon bonds increase (albeit only slightly) in length from 1.362 to 1.378 A (CCSD results). In striking contrast, both MP2 and DFT (B3LYP and PBE0) calculations fail in reproducing the pattern of the carbon-carbon bond lengths obtained with the CCSD, QCISD, and MP4 methods. The structures of three shorter n-even chains, C(n)H(-) (n = 2, 4, and 6), along with those of four n-odd compounds (n = 3, 5, 7, and 9) are also investigated at the CCSD/6-311+G(2d,p) level of theory.     

Electronic structure and chain-length effects in diplatinum polyynediyl complexes trans,trans-[(X)(R3P)2Pt(C triple bond C)(n)Pt(PR3)2(X)]: a computational investigation

Structure and bonding in the title complexes are studied using model compounds trans,trans-[(C6H5)(H3P)2Pt(C triple bond C)(n)Pt(PH3)2(C6H5)] (PtCxPt; x = 2n = 4-26) at the B3LYP/LACVP* level of density functional theory. Conformations in which the platinum square planes are parallel are very slightly more stable than those in which they are perpendicular (DeltaE = 0.12 kcal mol(-1) for PtC8Pt). As the carbon-chain length increases, progressively longer C triple bond C triple bonds and shorter triple bond C-C triple bond single bonds are found. Whereas the triple bonds in HCxH become longer (and the single bonds shorter) as the interior of the chain is approached, the PtC triple bond C triple bonds in PtCxPt are longer than the neighboring triple bond. Also, the Pt-C bonds are shorter at longer chain lengths, but not the H-C bonds. Accordingly, natural bond orbital charge distributions show that the platinum atoms become more positively charged, and the carbon chain more negatively charged, as the chain is lengthened. Furthermore, the negative charge is localized at the two terminal C triple bond C atoms, elongating this triple bond. Charge decomposition analyses show no significant d-pi* backbonding. The HOMOs of PtCxPt can be viewed as antibonding combinations of the highest occupied pi orbital of the sp-carbon chain and filled in-plane platinum d orbitals. The platinum character is roughly proportional to the Pt/Cx/Pt composition (e.g., x = 4, 31 %; x = 20, 6 %). The HOMO and LUMO energies monotonically decrease with chain length, the latter somewhat more rapidly so that the HOMO-LUMO gap also decreases. In contrast, the HOMO energies of HCxH increase with chain length; the origin of this dichotomy is analyzed. The electronic spectra of PtC4Pt to PtC10Pt are simulated. These consist of two pi-pi* bands that redshift with increasing chain length and are closely paralleled by real systems. A finite HOMO-LUMO gap is predicted for PtCinfinityPt. The structures of PtCxPt are not strictly linear (average bond angles 179.7 degrees -178.8 degrees ), and the carbon chains give low-frequency fundamental vibrations (x = 4, 146 cm(-1); x = 26, 4 cm(-1)). When the bond angles in PtC12Pt are constrained to 174 degrees in a bow conformation, similar to a crystal structure, the energy increase is only 2 kcal mol(-1). The above conclusions should extrapolate to (C triple bond C)(n) systems with other metal endgroups.     

A microwave and quantum chemical study of (trifluoromethyl)thiolacetic acid, CF3COSH, a compound with an unusual double-minimum potential

The microwave spectra of CF3COSH and one deuterated species, CF3COSD, have been investigated by Stark spectroscopy in the 40-80 GHz spectral range at -78 degrees C and by quantum chemical calculations using the HF, MP2, and B3LYP procedures with the aug-cc-pVTZ basis set. The microwave spectrum of one conformer was assigned. The conformations of the COSH and CF3 groups determine the overall conformation of this rotamer. It was not possible experimentally to find precise values for the associated dihedral angles, but it appears that the COSH group is distorted somewhat from an exact synperiplanar arrangement, while the CF3 group is rotated several degrees from a position where one of the C-F bonds eclipses the C-S bond. This rotamer tunnels through a transition state that has an exact Cs symmetry, where one C-F bond eclipses the C-S bond and the COSH group is synperiplanar. Relative intensity measurements yielded 28(15) cm-1 for the tunneling frequency. Two additional vibrationally excited states were assigned and their frequencies determined to be 94(30) and 184(40) cm-1, respectively. The theoretical calculations predict conflicting conformational properties for the identified rotamer. The B3LYP calculations find an exact synperiplanar arrangement for the COSH group, whereas the MP2 and HF calculations predict that this group is distorted slightly form this conformation. One of the C-F bonds is found to eclipse the C-S bond in the B3LYP calculations, while the MP2 calculations predict a slight deviation and the HF calculations a large deviation from the eclipsed position, as the corresponding F-C-C-S dihedral angle is calculated to be 0.9 degrees (MP2) and 27.6 degrees (HF). All three methods of calculations predict that a second rotamer coexists with the identified form but is several kJ/mol less stable. The spectrum of this form, which has overall Cs symmetry and is predicted to have an antiperiplanar conformation for the COSH group with one of the C-F bonds eclipsing the C=O bond, was not identified.     

The structure and chemical bonding in the N(2)-CuX and N(2)...XCu (X = F, Cl, Br) systems studied by means of the molecular orbital and Quantum Chemical Topology methods

Ab initio studies carried out at the MP2(full)/6-311+G(2df) and MP2(full)/aug-cc-pVTZ-PP computational levels reveals that dinitrogen (N(2)) and cuprous halides (CuX, X = F, Cl, Br) form three types of systems with the side-on and end-on coordination of N(2): N[triple bond]N-CuX (C(infinity v)), N(2)-CuX (C(2v)) stabilized by the donor-acceptor bonds and weak van der Waals complexes N(2)...XCu (C(2v)) with dominant dispersive forces. An electron density transfer between the N(2) and CuX depends on type of the N(2) coordination and a comparison of the NPA charges yields the [N[triple bond]N](delta+)-[CuX](delta-) and [N(2)](delta-)-[CuX](delta+) formula. According to the NBO analysis, the Cu-N coordinate bonds are governed by predominant LP(N2)-->sigma*(Cu-X) "2e-delocalization" in the most stable N[triple bond]N-CuX systems, meanwhile back donation LP(Cu)-->pi*(N-N) prevails in less stable N(2)-CuX molecules. A topological analysis of the electron density (AIM) presents single BCP between the Cu and N nuclei in the N[triple bond]N-CuX, two BCPs corresponding to two donor-acceptor Cu-N bonds in the N(2)-CuX and single BCP between electron density maximum of the N[triple bond]N bond and halogen nucleus in the van der Waals complexes N(2)...XCu. In all systems values of the Laplacian nabla(2)rho(r)(r(BCP)) are positive and they decrease following a trend of the complex stability i.e. N[triple bond]N-CuX (C(infinity v)) > N(2)-CuX (C(2v)) > N(2)...XCu (C(2v)). A topological analysis of the electron localization function (ELF) reveals strongly ionic bond in isolated CuF and a contribution of covalent character in the Cu-Cl and Cu-Br bonds. The donor-acceptor bonds Cu-N are characterized by bonding disynaptic basins V(Cu,N) with attractors localized at positions corresponding to slightly distorted lone pairs V(N) in isolated N(2). In the N[triple bond]N-CuX systems, there were no creation of any new bonding attractors in regions where classically the donor-acceptor bonds are expected and there is no sign of typical covalent bond Cu-N with the bonding pair. Calculations carried out for the N[triple bond]N-CuX reveal small polarization of the electron density in the N[triple bond]N bond, which is reflected by the bond polarity index being in range of 0.14 (F) to 0.11 (Cl).     

Gas phase infrared spectrum and ab initio calculations of phosphorus(III) thiocyanide, SPCN

An attempt has been made to record the gas phase infrared spectrum of phosphorus(III) thiocyanide, SPCN, for the first time. The molecule was generated by an on-line process using phosphorus(III) thiochloride, SPCl, as a precursor passed over heated silver cyanide at about 350 degrees C. The products were characterized by the infrared spectra of their vapors. The low resolution gas phase Fourier transform infrared spectrum shows three of six characterized fundamental modes of SPCN within the range of the spectrometer used at 2151, 743 and 622 cm(-1) These three bands were assigned to nu(1)(C[triple bond]N stretch), nu(2)(S=P stretch), and nu(3)(C--P stretch), respectively. Ab initio self-consistent-field (SCF) molecular orbital (MO) and M?ller-Plesset second-order perturbation theory (MP2) calculations were performed to determine the geometry, total energy and the vibrational frequencies of SPCN.     

The vibration-rotation emission spectrum of hot BeF2

The high-resolution infrared emission spectrum of BeF2 vapor at 1000 degrees C was rotationally analyzed with the assistance of large-scale ab initio calculations using the coupled-cluster method including single and double excitations and perturbative inclusion of triple excitations, in conjunction with correlation-consistent basis sets up to quintuple-zeta quality. The nu3 fundamental band, the nu1+nu2, nu1+nu3, and 2nu2+nu3 combination bands, and 18 hot bands were assigned. The symmetric stretching (nu1), bending (nu2), and antisymmetric stretching (nu3) mode frequencies were determined to be 769.0943(2), 342.6145(3), and 1555.0480(1) cm-1, respectively, from the band origins of the nu3, nu1+nu3, and nu1+nu2 bands. The observed vibrational term values and B rotational constants were fitted simultaneously to an effective Hamiltonian model with Fermi resonance taken into account, and deperturbed equilibrium vibrational and rotational constants were obtained for BeF2. The equilibrium rotational constant (Be) was determined to be 0.235 354(41) cm-1, and the associated equilibrium bond distance (re) is 1.3730(1) A. The results of our ab initio calculations are in remarkably good agreement with those of our experiment, and the calculated value was 1.374 A for the equilibrium bond distance (re). As in the isoelectronic CO2 molecule, the Fermi resonance in BeF2 is very strong, and the interaction constant k122 was found to be 90.20(4) cm-1.     

Aluminacyclopropene: syntheses, characterization, and reactivity toward terminal alkynes

Reactions of LAl with ethyne, mono- and disubstituted alkynes, and diyne to aluminacyclopropene LAl[eta2-C2(R1)(R2)] ((L = HC[(CMe)(NAr)]2, Ar = 2,6-iPr2C6H3); R1 = R2 = H, (1); R1 = H, R2 = Ph, (2); R1 = R2 = Me, (3); R1 = SiMe3, R2 = C[triple bond]CSiMe3, (4)) are reported. Compounds 1 and 2 were obtained in equimolar quantities of the starting materials at low temperature. The amount of C2H2 was controlled by removing an excess of C2H2 in the range from -78 to -50 degrees C. Compound 4 can be alternatively prepared by the substitution reaction of LAl[eta2-C2(SiMe3)2] with Me3SiC[triple bond]CC[triple bond]CSiMe3 or by the reductive coupling reaction of LAlI2 with potassium in the presence of Me3SiC[triple bond]CC[triple bond]CSiMe3. The reaction of LAl with excess C2H2 and PhC[triple bond]CH (<1:2) afforded the respective alkenylalkynylaluminum compounds LAl(CH=CH2)(C[triple bond]CH) (5) and LAl(CH=CHPh)(C[triple bond]CPh) (6). The reaction of LAl(eta2-C2Ph2) with C2H2 and PhC[triple bond]CH yielded LAl(CPh=CHPh)(C[triple bond]CH) (7) and LAl(CPh=CHPh)(C[triple bond]CPh) (8), respectively. Rationally, the formation of 5 (or 6) may proceed through the corresponding precursor 1 (or 2). The theoretical studies based on DFT calculations show that an interaction between the Al(I) center and the C[triple bond]C unit needs almost no activation energy. Within the AlC2 ring the computational Al-C bond order of ca. 1 suggests an Al-C sigma bond and therefore less pi electron delocalization over the AlC2 ring. The computed Al-eta2-C2 bond dissociation energies (155-82.6 kJ/mol) indicate a remarkable reactivity of aluminacyclopropene species. Finally, the 1H NMR spectroscopy monitored reaction of LAl(eta2-C2Ph2) and PhC[triple bond]CH in toluene-d8 may reveal an acetylenic hydrogen migration process.     

Hydrogen bond vibrations of 2-aminopyridine.2-pyridone,a Watson-Crick analogue of adenine.uracil

The 2-aminopyridine.2-pyridone complex (2AP.2PY) is linked by antiparallel N-H.O=C and N-H.N hydrogen bonds, providing a model for the Watson-Crick hydrogen bond configuration of the adenine-thymine and adenine-uracil nucleobase pairs. Mass-selected S(1) <--> S(0) vibronic spectra of the supersonically cooled 2AP.2PY base pair analogue were measured by laser resonant two-photon ionization and emission spectroscopies. The hydrogen bond vibrations nu(2) (buckle, out-of-plane) and the three in-plane vibrations nu(3) (opening), nu(5) (shear), and nu(6) (stretch) were observed in the S(0) and S(1) states, giving detailed information on the stretching and deformation force constants of the (amide)N-H.N(pyridine) and the (amino)N-H.O=C hydrogen bonds. Density functional calculations with the B3LYP functional and the 6-311++G(d,p) and 6-311++G(2d,2p) basis sets yield ground-state hydrogen bond frequencies in close agreement with experiment.     

A microwave spectroscopic and quantum chemical study of propa-1,2-dienyl selenocyanate (H(2)==C==CHSeC[triple bond]N) and cyclopropyl selenocyanate (C(3)H(5)SeC[triple bond]N)

The microwave spectra of propa-1,2-dienyl selenocyanate, H(2)C==C==CHSeC[triple bond]N, and cyclopropyl selenocyanate, C(3)H(5)SeC[triple bond]N, are reported. The spectra of the ground and two vibrationally excited states of the (80)Se isotopologue and the spectrum of the ground state of the (78)Se isotopologue were assigned for one rotameric form of H(2)C==C[double bond, length as m-dash]CHSeC[triple bond]N. This conformer is characterized by a C-C-Se-C dihedral angle of 129(5) degrees from synperiplanar (0 degrees ) and is shown to be the global minimum of H(2)C[double bond, length as m-dash]C[double bond, length as m-dash]CHSeC[triple bond]N. The spectra of the ground and of three vibrationally excited states of the (80)Se isotopologue, as well as of the ground state of the (78)Se isotopologue of one rotamer of C(3)H(5)SeC[triple bond]N were assigned. This conformer has a H-C-Se-C dihedral angle of 80(4) degrees from synperiplanar and is at least 3 kJ mol(-1) more stable than any other form of the molecule. The microwave study has been augmented by quantum chemical calculations at the B3LYP/6-311+ +G(3df,3pd) and MP2/6-311+ +G(3df,3pd) levels of theory.     

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.     

Tracking the chemistry of unsaturated C3H3 groups adsorbed on a silver surface: propargyl-allenyl-acetylide triple bond migration, self-hydrogenation, and carbon-carbon bond formation

A diverse array of unsaturated C1 (methylene and methylidyne) and C2 (vinyl, vinylidene, ethylidene, and ethylidyne) bound to metal center(s) and surfaces has received much attention. In sharp contrast to the effort devoted to C1 and C2 ligands, complexes or surfaces bearing C3 fragments have been less explored, especially the M-C3H3 systems, which include propargyl (M-CH2C[triple bond]CH), allenyl (M-CH=C=CH2), and acetylide (M-C[triple bond]CCH3) forms. To understand the bonding and reactivity of these C3 species appended to an extended metal structure, proprargyl bromide (Br-CH2C[triple bond]CH) was utilized as a precursor to generate C3H3 fragments on a Ag(111) surface under ultrahigh vacuum conditions. The molecular transformation process was explored by a combination of temperature-programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and X-ray photoemission spectroscopy (XPS) techniques. In addition, density functional theory (DFT) calculations were conducted to obtain the optimized geometries and energies for the various surface intermediates. The computed IR spectra facilitated the vibrational mode assignments. TPD spectra show that C3H3(ad) self-hydrogenates to C3H4 around 300 and 475 K, respectively. In addition to hydrogenation, a C-C coupling product C6H6 (2,4-hexadiyne) is also unveiled as part of the desorption feature at 475 K. Identification of the possible C3H4 isomers (propyne and/or allene) was equivocal, but it was circumvented by using an alpha,alpha-dimethyl-substituted propargylic species--(CH3)2(alpha)C-C[triple bond]CH, which results in hydrogenation products, alkynic (CH3)2CH-C[triple bond]CH and allenic (CH3)2C=C=CH2, distinguishable by the mass spectrometry. The substitution experiments clarify that in the normal case the convoluted TPD feature around 300 K, in fact, consists of both allene at 260 K and propyne at 310 K, while the last hydrogenation product at 475 K is solely propyne. The RAIR spectroscopy demonstrates that at 200 K C3H3(ad) on Ag(111) readily adopts the allenyl formalism involving concerted CBr bond scission and [1,3]-sigmatropic migration (i.e., Br-*CH2C[triple bond]CH --> *CH2=C=CH-Ag), in which the sigma bond moves to a new metal location across the pi-periphery. Single hydrogen incorporation to the alpha-carbon of the surface allenyl rationalizes the allene formation at 260 K. When the surface is heated to the range of 250-300 K, both RAIR and XP spectra reveal drastic changes, indicative of a new species whose spectral characteristics could be duplicated by separate measurements from 1-propyn-1-yl iodide (CH3-C[triple bond]C-I) being a direct source for the surface methylacetylide (CH3-C[triple bond]C-Ag). It is thus suggested that allenyl is further reorganized to render acetylide presumably via [1,3]-hydrogen shift (i.e., *CH2=C=CH-Ag --> *CH3=C[triple bond]C-Ag). The presence of this third Ag-C3H3 isomeric form demonstrates an unprecedented propargyl-allenyl-acetylide multiple rearrangements on a metal surface. Migration of the triple bond from the remote terminal position into the chain, through the stage of allenic structure, is driven by thermodynamic stabilities, supported by the DFT total energy calculations. Consequently, the evolutions of propyne at 310 and 475 K, as well as 2,4-hexadiyne (bismethylacetylide), can all be reasoned out.