Vibronic spectroscopy of benzyl-type radicals: observation of mesityl radical in the gas phase

We observed, for the first time, the vibronic emission spectrum of the jet-cooled mesityl radical that was formed from mesitylene seeded in a large amount of inert carrier gas helium using a pinhole-type glass nozzle in a corona excited supersonic expansion. The well-resolved vibronic emission spectrum was recorded in the visible region with a long path monochromator. The spectrum was analyzed to identify the origin of the D(2) --> D(0) and D(1) --> D(0) transitions as well as the frequencies of the vibrational modes in the ground electronic state of the mesityl radical by comparison with those of the known data of the precursor and an ab initio calculation.     

Vibronic coupling in the benzyl radical

The vibronic perturbations among the lowest excited states of benzyl, computed by a CNDO/S program in the floating-orbital scheme, are presented. The ν_8b (ν_18b) modes are the vibrations that most strongly (weakly) couple the quasidegenerate 1A₂ and 2B₂ states. The results are relevant to the interpretation of benzyl emission and absorption spectra.     

Generation and detection of the peroxyacetyl radical in the pyrolysis of peroxyacetyl nitrate in a supersonic expansion

The peroxyacetyl radical (PA, CH3C(O)OO) is generated by flash pyrolysis of peroxyacetyl nitrate (PAN, CH3C(O)OONO2) in a supersonic jet. The 0(0)(0) A2A' <-- X2A' electronic transition for PA, at ca. 5582 cm(-1), is detected in a supersonically cooled sample by time-of-flight mass spectroscopy in the CH3CO mass channel. Rotational envelope simulation results find that the rotational temperature for PA in its ground electronic and vibrational state is ca. 55 K. At ca. 330 degrees C, the thermal decomposition of PAN by flash pyrolysis in a heated nozzle with supersonic expansion is mainly by formation of PA and NO2. The maximum yield of PA is obtained at this temperature. At higher temperatures (300-550 degrees C), an intense signal in the CH2CO+ mass channel is observed, generated by the decomposition of PA.     

Vibronic interactions in the photodetachment spectroscopy of phenide anion

Photodetachment spectroscopy of phenide anion C6H5- is theoretically studied with the aid of electronic structure calculations and quantum dynamical simulations of nuclear motion. The theoretical results are compared with the available experimental data. The vibronic structure of the first, second, and third photoelectron bands associated with the ground X 2A1 and low-lying excited A 2B1 and B 2A2 electronic states of the phenyl radical C6H5 is examined at length. While the X state of the radical is energetically well separated and its interaction is found to be rather weak with the rest, the A and B electronic states are found to be only approximately 0.57 eV apart in energy at the vertical configuration. Low-energy conical intersections between the latter two states are discovered and their impact on the nuclear dynamics underlying the second and third photoelectron bands is delineated. The nuclear dynamics in the X state solely proceeds through the adiabatic path and the theoretically calculated vibrational level structure of this state compares well with the experimental result. Two Condon active totally symmetric (a1) vibrational modes of ring deformation type form the most dominant progression in the first photoelectron band. The existing ambiguity in the assignment of these two vibrational modes is resolved here. The A-B conical intersections drive the nuclear dynamics via nonadiabatic paths, and as a result the second and third photoelectron bands overlap and particularly the third band due to the B state of C6H5 becomes highly diffused and structureless. Experimental photodetachment spectroscopy results are not available for these bands. However, the second band has been detected in electronic absorption spectroscopy measurements. The present theoretical results are compared with these absorption spectroscopy data to establish the nonadiabatic interactions between the A and B electronic states of C6H5.     

Nitrate radical quantum yield from peroxyacetyl nitrate photolysis

Peroxyacetyl nitrate (PAN, CH3C(O)OONO2) is a ubiquitous pollutant that is primarily destroyed by either thermal or photochemical mechanisms. We have investigated the photochemical destruction of PAN using a combination of laser pulsed photolysis and cavity ring-down spectroscopic detection of the NO3 photoproduct. We find that the nitrate radical quantum yield from the 289 nm photolysis of PAN is Phi(NO3)PAN = 0.31 +/- 0.08 (+/-2 sigma). The quantum yield is determined relative to that of dinitrogen pentoxide, which is assumed to be unity, under identical experimental conditions. The instrument design and experimental procedure are discussed as well as auxiliary experiments performed to further characterize the performance of the optical cavity and photolysis system.     

The analysis of the near IR of some organic and organometallic compounds by photoacoustic spectroscopy

The photoacoustic spectrum (PAS) was measured in the near IR region (1000 to 2600 nm) for organic compounds (C₆H₆, C₆D₆, C₇H₈, and C₆H₁₂) and organometallic compounds (Cp₂Fe, Cp₂Fe₂(CO)₄, Cp₄Fe₄(CO)₄, Cp₄Fe₄S₄, C₆H₆Cr(CO)₃, CpCo(C₄Ph₄) and CpCo(CO)₂). Band assignments were made by comparison to the infrared spectra. The bands were assigned as the CH overtone stretch and combinations of CH and other IR fundamentals. These bands provide fingerprint spectra for these compounds.     

Vibronic structure of jet-cooled 2,6-dimethylbenzyl radical: revisited

We reexamined the vibronic structure of the jet-cooled 2,6-dimethylbenzyl radical that was generated from 1,2,3-trimethylbenzene seeded in a large amount of inert carrier gas helium using a pinhole-type glass nozzle in a corona excited supersonic expansion, from which the vibronically resolved emission spectrum was recorded with a long path double monochromator in the visible region. The spectrum exhibited bands arising from not only the D1 --> D0 transition but also the D2 --> D0 transition, in which transitions the accurate electronic energies of the D2 and D1 states and the revised vibrational mode frequencies in the ground electronic state were obtained by comparison with those from the known data of the precursor and an ab initio calculation.     

Vibronic dynamics in the low-lying coupled electronic states of methyl cyanide radical cation

Static and dynamic aspects of the Jahn–Teller (JT) and pseudo-Jahn–Teller (PJT) interactions between the ground and first excited electronic states of the methyl cyanide radical cation are theoretically investigated here. The latter involves construction of a theoretical model by ab initio computation of electronic potential energy surfaces and their coupling surfaces and simulation of the nuclear dynamics employing time-independent and time-dependent quantum mechanical methods. The present system represents yet another example belonging to the (E + A)  e JT–PJT family, with common JT and PJT active degenerate (e) vibrational modes. The theoretical results are found to be in very good accord with the recent experimental data revealing that the JT interactions are particularly weak in the ground electronic manifold of methyl cyanide radical cation, On the other hand, the PJT interactions of this ground electronic manifold with the first excited electronic state of the radical cation are stronger which cause an increase of the spectral line density. The effect of deuteration on the JT–PJT dynamics of the methyl cyanide radical cation is also discussed.     

Fourier self-deconvolution in IR spectroscopy

Fourier self-deconvolution (FSD) is a mathematical means for reducing bandwidths, so that overlapped bands can be resolved from one another. The principles of FSD are described briefly, and examples are shown of how overlapped infrared spectra can be enhanced so as to greatly improve their information content. The disadvantages of FSD are discussed and it is shown how the method can be extended to extract individual components from a composite envelope of bands.     

Slit discharge IR spectroscopy of a jet-cooled cyclopropyl radical: structure and intramolecular tunneling dynamics

High-resolution infrared spectra of a jet-cooled cyclopropyl radical are reported for the first time, specifically sampling the in-phase antisymmetric CH2 stretch (nu7) vibration. In addition to yielding the first precise gas-phase structural information, the spectra reveal quantum level doubling into lower (+) and upper (-) states due to tunneling of the lone alpha-CH with respect to the CCC plane. The bands clearly reveal intensity alternation due to H atom nuclear spin statistics (6:10 and 10:6 for even:odd Ka+Kc in lower (+) and upper (-) tunneling levels, respectively) consistent with C2v symmetry of the cyclopropyl-tunneling transition state. The two ground-state-tunneling levels fit extremely well to a rigid asymmetric rotor Hamiltonian, but there is clear evidence for both local and global state mixing in the vibrationally excited nu7 tunneling levels. In particular, the upper (-) tunneling component of the nu7 state is split by anharmonic coupling with a nearly isoenergetic dark state, which thereby acquires oscillator strength via intensity sharing with this bright state. From thermal Boltzmann analysis of fractional populations, tunneling splittings for a cyclopropyl radical are estimated to be 3.2 +/- 0.3 cm(-1) and 4.9 +/- 0.3 cm(-1) in the ground and nu7-excited states, respectively. This analysis indicates ground-state stereoracemization of the alpha-CH radical center to be a very fast process [k approximately 2.0(4) x 10(11) s(-1)], with the increase in the tunneling rate upon CH2 in-phase asymmetric stretch excitation consistent with ab initio predictions of equilibrium vs transition-state zero-point energies. Modeling of the ground-state-tunneling splittings with high level ab initio 1D potentials indicates an improved V0 = 1115 +/- 35 cm(-1) barrier height for alpha-CH inversion through the cyclopropyl CCC plane.     

Vibronic spectroscopy of single C60 molecules and monolayers with the STM

A scanning tunneling microscope is used to study the differential conductance (dI/dV) of single C(60) molecules in isolation and in monolayers adsorbed on NiAl(110) and on an ultrathin alumina film grown on the NiAl(110) surface. On the oxide layer, the electronic bands in the dI/dV spectra display a series of equally spaced features, attributed to the vibronic states of the molecules, which are absent when the molecules are adsorbed on the metal. A comparison between the molecular spectra measured on the oxide film reveals the effect of adsorption temperature and geometry, as well as intermolecular interactions on the vibronic features.     

In situ Fourier transform near infrared spectroscopy monitoring of copper mediated living radical polymerization

In situ Fourier transform near infrared (FTNIR) spectroscopy was successfully used to monitor monomer conversion during copper mediated living radical polymerization with N‐(n‐propyl)‐2‐pyridylmethanimine as a ligand. The conversion of vinyl protons in methacrylic monomers (methyl methacrylate, butyl methacrylate, and N‐hydroxysuccinimide methacrylate) to methylene protons in the polymer was monitored with an inert fiber‐optic probe. The monitoring of a poly(butyl methacrylate‐b‐methyl methacrylate‐b‐butyl methacrylate) triblock copolymer has also been reported with difunctional poly(methyl methacrylate) as a macroinitiator. In all cases FTNIR results correlated excellently with those obtained by ¹H NMR. On‐line near infrared (NIR) measurement was found to be more accurate because it provided many more data points and avoided sampling during the polymerization reaction. It also allowed the determination of kinetic parameters with, for example, the calculation of an apparent first‐order rate constant. All the results suggest that FTNIR spectroscopy is a valuable tool to assess kinetic data. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4933–4940, 2004     

Direct IR spectroscopic study of the cyclopentadienyl radical

The products of vacuum pyrolysis of bis(cyclopentadienyl)nickel have been studied by matrix isolation infrared spetroscopy. Three IR bands at 3079, 1383 and 661 cm^–1 corresponding to the cyclopentadienyl radical have been observed. A comparison of the IR spectrum of the radical C₅H₅ with those of the ligand -C₅H₅ and free anion^–C₅H₅ has been performed.These results were presented for the first time in O. M. Nefedov's plenary lecture at X IUPAC Conference on Physical Organic Chemistry (Haifa, Israel, August 1990) (see ref. 1 and literature cited therein).Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1497–1499, August, 1993.     

IR spectroscopy for unstable molecules

Summary Infrared spectroscopy is a rather old-fashioned analytical technique; however, developments over the past few years in both Fourier Transform IR and IR laser techniques have permitted IR spectroscopy to be applied increasingly successfully to the detection, characterisation and monitoring of unstable molecules. Three experimental techniques, each of which employs principally IR spectroscopy, are described: matrix isolation; liquid noble gases as solvents; time-resolved studies. Examples given concentrate on organometallic intermediates of possible relevance to homogeneous catalysis.

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IR-Spektroskopie für instabile Moleküle

Zusammenfassung Infrarot-Spektroskopie ist eine ziemlich altmodische analytische Technik; jedoch haben es die Entwicklungen der letzten Jahre in der Fourier-Transform-IR-Interferometrie und in der IR-Laser-Technik ermöglicht, die IR-Spektroskopie mit wachsendem Erfolg beim Nachweis, bei der Charakterisierung und bei der Beobachtung instabiler Moleküle einzusetzen. Drei experimentelle Techniken, von denen jede auf der IR-Spektroskopie basiert, werden beschrieben: Matrix-Isolation, flüssige Edelgase als Lösungsmittel und zeitaufgelöste Untersuchungen. Die vorgestellten Beispiele konzentrieren sich auf möglicherweise für die homogene Katalyse wesentliche metallorganische Zwischenprodukte.

     

Bulk radical homo-polymerisation studies of commercial acrylate monomers using near infrared Fourier transform Raman spectroscopy

In our previous work we have used near infrared Fourier transform Raman spectroscopy to provide a reliable, reproducible and quantitative method for in situ reaction monitoring of homo-polymerisation. Model systems such as styrene and methylmethacrylate were studied. In this present work three commercial monomers have been studied, namely butylacrylate, hydroxylpropylmethacrylate and laurylmethacrylate. The effects of reaction temperature, monomer additives and the presence of oxygen have been elucidated. These reaction variables have been shown to have a significant effect on polymerisation rate in particular for the butylacrylate system.     

Polyene spectroscopy: Vibronic evidence for a forbidden transition in DECA-2,4,6,8-tetraene

The dimethylpolyene deca-2,4,6,8-tetraene was studied by absorption, fluorescence excitation and fluorescence spectroscopy in glasses at 77 K and in n-alkane crystals at 4.2 K. A strong transition to a ¹B_u excited state is observed with an origin at 32400 cm^?1 in isopentane at 77 K and at 31280 cm^?1 in n-undecane at 4.2 K. A weak transition to a ¹A_g excited state is observed with an origin at 28738 cm^?1 in the n-undecane matrix. The radiative fluorescence lifetime is 500 ns. In undecane the transition from the ground state to the ¹A_g excited state exhibits a classic Herzberg—Teller vibronic pattern indicating a symmetry forbidden transition.     

IR spectroscopy investigation of molecular association in the acetone-trichlormethane mixture

A significant increase in the stretching band intensity of trichloromethane in IR spectra of its solutions in acetone with increasing acetone mole fraction indicates the formation of hydrogen bonds between their molecules. The integral absorption coefficient (α) of this vibration is calculated. Experimental values of α are approximated by a theoretical dependence using the nonlinear least-squares method; association parameters are obtained. The relative volume fractions of H-bonded and free trichloromethane molecules are calculated depending on the total trichloromethane volume fraction in solution. The structure of the H-bonded associate is determined by quantum chemical calculations.     

Investigation of the conformation of epoxychalcones by IR spectroscopy

Molecules of ketone oxides in the solid phase exist exclusively in the gauche form, whereas compounds of the open type form a mixture of cis and gauche conformers in solution. The effect of temperature on the conformational equilibrium was investigated.