Karsten Krogh-Jespersen

The glycine zwitterion does not exist in the gas phase: results from a detailed ab initio electronic structure study

Abstract We have computed the properties of the zwitterionic form of glycine (NH + 3 CH 2 COO − ) using ab initio molecular orbital theory. At the Hartree—Fock level, a shallow minimum on the zwitterion potential energy surface may be located with certain basis sets that do not contain p-type polarization functions on hydrogen. The small barriers ( 2 CH 2 COOH) disappear, however, when corrections are made for vibrational zero point energies or electronic correlation. The zwitterion is not a minimum at the Hartree—Fock level when the basis set contains hydrogen polarization functions nor when correlated wavefunctions are employed.

Net Oxidative Addition of C(sp3)-F Bonds to Iridium via Initial C-H Bond Activation

An unusual mechanism to cleave carbon-fluorine bonds may facilitate more efficient transformations of fluorocarbons. Carbon-fluorine bonds are the strongest known single bonds to carbon and as a consequence can prove very hard to cleave. Alhough vinyl and aryl C-F bonds can undergo oxidative addition to transition metal complexes, this reaction has appeared inoperable with aliphatic substrates. We report the addition of C(sp3)-F bonds (including alkyl-F) to an iridium center via the initial, reversible cleavage of a C-H bond. These results suggest a distinct strategy for the development of catalysts and promoters to make and break C-F bonds, which are of strong interest in the context of both pharmaceutical and environmental chemistry.

The 1:1 glycine zwitterion‐water complex: An ab initio electronic structure study

More than a dozen stationary points on the potential energy surface for the 1:1 glycine zwitterion—water complex have been investigated at Hartree‐Fock or MP2 levels of theory with basis sets ranging from split valence (4‐31G) to split valence plus polarization and diffuse function (6–31 + + G**) quality. Only one true minimum (GLYZWM, C1 symmetry) could be located on the potential energy surface. GLYZWM features a bridged water molecule acting as both a hydrogen bond acceptor and donor with the NH3− and CO2− units of the glycine zwitterion. The total hydrogen bond energy in GLYZWM is computed as 16 kcal/mol (MP2/6–31 ++ G** // 6–31 ++ G**, including corrections for basis set superpositions errors). The computed vibrational frequencies and normal mode forms of the GLYZWM complex resemble in many cases experimental assignments made for the glycine zwitterion in bulk water on the basis of Raman spectroscopy.

Multiphoton ionization photoelectron spectroscopy of phenol: Vibrational frequencies and harmonic force field for the 2B1 cation

A molecular beam of phenol, cooled by a supersonic expansion, is crossed at right angles by the output of a pulsed frequency‐doubled dye laser, causing 1+1 resonance enhanced multiphoton ionization. The kinetic energy of the resulting photoelectrons is determined as a function of laser wavelength with time‐of‐flight analysis, permitting the assignment of 11 vibrational frequencies for the 2B1 phenol‐h6 cation and ten vibrational frequencies for phenol‐d5. Of these, all but the lowest frequency one in each case are in‐plane vibrations of which phenol has a total of 19. An approximate harmonic force field for the in‐plane modes of the phenol cation is derived along with its associated frequencies and mode forms. This in turn facilitates the vibrational analysis. Analogous force field calculations have been carried out on the ground (1A1) and first excited (1B2) states of the neutral parent, permitting conclusions to be reached concerning bonding changes upon removal of an electron from the phenol electron s...

Cleavage of sp3 C-O Bonds via Oxidative Addition of C-H Bonds

(PCP)Ir (PCP = kappa(3)-C(6)H(3)-2,6-[CH(2)P(t-Bu)(2)](2)) is found to undergo oxidative addition of the methyl-oxygen bond of electron-poor methyl aryl ethers, including methoxy-3,5-bis(trifluoromethyl)benzene and methoxypentafluorobenzene, to give the corresponding aryloxide complexes (PCP)Ir(CH(3))(OAr). Although the net reaction is insertion of the Ir center into the C-O bond, density functional theory (DFT) calculations and a significant kinetic isotope effect [k(CH(3))(OAr)/k(CD(3))(OAr) = 4.3(3)] strongly argue against a simple insertion mechanism and in favor of a pathway involving C-H addition and alpha-migration of the OAr group to give a methylene complex followed by hydride-to-methylene migration to give the observed product. Ethoxy aryl ethers, including ethoxybenzene, also undergo C-O bond cleavage by (PCP)Ir, but the net reaction in this case is 1,2-elimination of ArO-H to give (PCP)Ir(H)(OAr) and ethylene. DFT calculations point to a low-barrier pathway for this reaction that proceeds through C-H addition of the ethoxy methyl group followed by beta-aryl oxide elimination and loss of ethylene. Thus, both of these distinct C-O cleavage reactions proceed via initial addition of a C(sp(3))-H bond, despite the fact that such bonds are typically considered inert and are much stronger than C-O bonds.

Vibronic mechanisms in the two‐photon spectrum of benzene

Intensity measurements have been made on the single quantum bands of the active modes in the two‐photon 1B2u←1A1g spectra of C6H6 and its isotopic homologs C6H5D, o‐C6H4D2, C6HD5, p‐C6H4D2, p‐C6H2D4, s‐C6H3D3, and C6D6. ν14 (b2u) and ν18 (e1u) (D6h symmetry) are found to be active in all the isotopic benzenes, and ν12 (b1u) in all except C6H6, s‐C6H3D3, and C6D6 where it is identity forbidden, and p‐C6H4D2 where it is weak. The results show that the band strength of 1410 is insensitive to deuterium substitution (confirming Duschinsky rotation of this mode), while 1210 and 1810 are strongly sensitive to the number and orientation of the deuterium atoms. INDO/S calculations of transition tensors at displaced nuclear coordinates, with and without doubly excited configurations, indicate that the ground state vibronic mechanisms usually neglected in one‐photon spectra are important in the benzene two‐photon spectra. The activity of ν14 can be attributed to vibronic perturbation of the final B2u state by the gr...

Covalent Bonding and the Trans Influence in Lanthanide Compounds

A pair of mer-octahedral lanthanide chalcogenolate coordination complexes [(THF)(3)Ln(EC(6)F(5))(3) (Ln = Er, E = Se; Ln = Yb, E = S)] have been isolated and structurally characterized. Both compounds show geometry-dependent bond lengths, with the Ln-E bonds trans to the neutral donor tetrahydrofuran (THF) significantly shorter than the Ln-E bonds that are trans to negatively charged EC(6)F(5) ligands. Density functional theory calculations indicate that the structural trans influence evidenced by the differences in these bond lengths results from a covalent Ln-E interaction involving ligand p and Ln 5d orbitals.

Two-photon ionization spectrum of the 1Lb ← S0 transition in toluene

Abstract The multiphoton ionization (MPI) spectrum of toluene arising from the 1 B 2 ( 1 L b ) valence state has been investigated. The state participates as a two-photon resonance. A total of nine excited state fundamentals have been characterized, including three non-totally symmetric vibrations. The toluene MPI spectrum shows a strong resemblance to the two-photon fluorescence excitation spectrum with the strongest transitions taking place to the origin and excited state modes ν 1 (a 1 ), ν 12 (a 1 ) and ν 14 (b) 2 ). The intensities of the observed fundamentals are rationalized in terms of Franck-Condon and vibronic coupling effects. A major conclusion is, that the primary mechanism for the activity of ν 12 is vibronic coupling.

Thermochemical alkane dehydrogenation catalyzed in solution without the use of a hydrogen acceptor

(PCP)IrH2 [PCP = η3-C6H3(PBut2)2-1,3] catalyzes the efficient (several hundred mol product/mol catalyst) dehydrogenation of alkanes under reflux to give the corresponding alkenes and dihydrogen.

Dehydrogenation of n-Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of α-Olefin Product.

We report the transfer-dehydrogenation of gas-phase alkanes catalyzed by solid-phase, molecular, pincer-ligated iridium catalysts, using ethylene or propene as hydrogen acceptor. Iridium complexes of sterically unhindered pincer ligands such as (iPr4)PCP, in the solid phase, are found to give extremely high rates and turnover numbers for n-alkane dehydrogenation, and yields of terminal dehydrogenation product (α-olefin) that are much higher than those previously reported for solution-phase experiments. These results are explained by mechanistic studies and DFT calculations which jointly lead to the conclusion that olefin isomerization, which limits yields of α-olefin from pincer-Ir catalyzed alkane dehydrogenation, proceeds via two mechanistically distinct pathways in the case of ((iPr4)PCP)Ir. The more conventional pathway involves 2,1-insertion of the α-olefin into an Ir-H bond of ((iPr4)PCP)IrH2, followed by 3,2-β-H elimination. The use of ethylene as hydrogen acceptor, or high pressures of propene, precludes this pathway by rapid hydrogenation of these small olefins by the dihydride. The second isomerization pathway proceeds via α-olefin C-H addition to (pincer)Ir to give an allyl intermediate as was previously reported for ((tBu4)PCP)Ir. The improved understanding of the factors controlling rates and selectivity has led to solution-phase systems that afford improved yields of α-olefin, and provides a framework required for the future development of more active and selective catalytic systems.