Jonathan E. Kenny

The photodissociation of van der Waals molecules: Complexes of iodine, neon, and helium

A supersonic free jet expansion was used to prepare van der Waals complexes of the type I2NeaHeb. Complexes containing as many as seven rare gas atoms were identified as satellites in the fluorescence excitation spectrum of the I2 B (v′=13 to 26) ←X (v″=0) band system by their relative dependence on the concentrations of neon and helium. For a+b?6, the frequencies of the van der Waals satellites follow a simple band shift rule: ν (I2NeaHeb) =ν (I2)+Aa+Bb, where A and B are weak functions of the I2 vibrational state. This observation, along with the failure of the rule for I2Ne7, provide some information concerning the geometry and binding in these molecules. Progressions (w′=0,1, and 2) in van der Waals modes of I2Ne and I2NeHe were also identified. The problem of intramolecular energy transfer was studied by observation of the dispersed emission spectra of the I2* fragments produced upon laser‐induced photodissociation of these complexes. The product vibrational state distributions could be determined by...

Vibrational predissociation and intramolecular vibrational relaxation in electronically excited s‐tetrazine–argon van der Waals complex

The van der Waals complex tetrazine–argon formed in a supersonic expansion has been investigated by means of laser‐induced excitation and fluorescence spectroscopy. Excitation spectra reveal the band shift D′′0−D′0=−23 cm−1, the tetrazine–argon stretch frequency νw=44 cm−1, and a lower limit to the dissociation energy D′′0≳254 cm−1. The band shift is vibrationally dependent in the case of two low frequency out‐of‐plane modes of tetrazine. The absence of additional broadening of the excitation linewidths due to the presence of argon shows that the dynamical processes in the complex are slower than the photodissociation of tetrazine itself (0.5–1.4 ns) which serves as an internal timing mechanism. At five single vibronic levels of tetrazine–argon above the S1 zero‐point level, the rates of vibrational predissociation and intramolecular vibrational relaxation are ∼108 s−1. The V→V propensities in dissociation and relaxation are selective and different from collision‐induced propensities. Among the V→V channe...

Improvement of Inner Filter Effect Correction Based on Determination of Effective Geometric Parameters Using a Conventional Fluorimeter

The most widely used correction of fluorescence intensities for inner filter effects in conventional (90 degrees ) fluorimeters fails at high absorbance values. We have critically examined this failure, which is caused by the difference between the geometrical parameters (GPs) of the excitation and emission beams in the typical instrument (focused beams) and in the theoretical picture on which the correction is based (collimated beams). We provide two types of experimental measurement of GPs and show that their substitution in the correction equations leads to significant improvements in the linear range of corrected fluorescence. We also demonstrate that mathematical optimizations give greater improvements and that the optimizations yield GPs consistent with experimental measurements. For solutions exhibiting primary inner filter effect only, we have extended the range of linearity of corrected fluorescence to a(ex) (absorbance per cm) up to 5.3; for systems with both primary and secondary inner filter effects we have achieved linearity for a(ex) + a(em) = 6.7. In all cases linear fits have slopes which agree well with the dilute limit. Different series of one- and two-solute solutions were used to demonstrate effectiveness of our correction methods. We also provide a rationale for the unexpected independence of GPs on excitation and emission bandwidths.

Free‐jet fluorescence excitation spectrum of trans, trans‐1,3,5,7‐octatetraene

The fluorescence excitation spectrum from 2980 to 2650 A of trans, trans‐1,3,5,7‐octatetraene in a free jet has been measured monitoring emission in the region 3250 to 3500 A. The vibronic bands of the 1 1Bu←1 1Ag (S2←S0) transition of octatetraene narrow significantly in a seeded free jet of ∼8 ppm octatetraene in helium (FWHM∼200 cm−1 for the room temperature static gas; FWHM∼35 cm−1 in the jet). Although absolute intensities of the vibronic bands are not determined, there is significant intensity in the origin, and the vibronic development involves only a few modes. Estimated translational, rotational, and vibrational temperatures of ∼1,<45, and <170 K, respectively, were achieved in the expansion. The major source of the observed linewidth is attributed to coupling of the 1 1Bu state to the lower 1Ag states.

Nonstatistical behavior in van der Waals photochemistry: Tetrazine–Ar

S‐tetrazine–Ar was prepared by free jet expansion of dilute mixture. Vibrational levels were excited by dye laser radiation and the resulting emission spectra analyzed. (AIP)

Dual fluorescence excitation spectra of methyl salicylate in a free jet

Separate fluorescence excitation spectra of the blue‐ and UV‐emitting forms of methyl salicylate cooled in a free‐jet expansion are reported. This study represents the first observation of the detailed vibrational structure of these transitions. The two excitation spectra have no features in common, and their intensity patterns are very different. Many individual lines are ∼2 cm−1 wide (nearly laser limited), although in the excitation spectrum of the UV emission, spectral congestion persists at high energies despite the high degree of cooling. (AIP)

Infrared radiative forcing of CFC substitutes and their atmospheric reaction products

We have employed the radiative forcing function recently computed by Pinnock et al. [1995], in conjunction with infrared cross sections and vibrational frequencies calculated with ab initio quantum-mechanical methods, to estimate the instantaneous infrared radiative forcing terms. F tot of a number of CFC substitutes and their atmospheric reaction products. The computed quantities F tot are compared with those obtained by Pinnock et al. [1995] employing experimental infrared spectra. In particular, Gaussian-92 TM software has been utilized, at the MP2/6-31G ** level of theory, to provide computational estimates of these quantities. Our results agree with the former, with a mean and a standard deviation of the difference of +3% and 15%, respectively. An error analysis is provided to assess the utility of the method. The capability of quantum-mechanical computational methods to explore structural trends in radiative and physicochemical properties is exploited for the molecular systems examined here.

Effects of steric crowding on porphyrin conformation. Synthesis, crystal structure and molecular stereochemistry of μ-oxo-bis[(5,15-dimethyl-2,3,7,8,12,13,17,18-octaethylporphinato)iron(III)]

Abstract The preparation and structural characterization of [Fe(ODM)]2O is described. This μ-oxo iron(III) derivative has a nearly eclipsed arrangement of the two porphinato cores when viewed along the nearly linear Fe-O-Fe spine. This feature and the pronounced nonplanarity of the porphinato core are the consequence of significant steric congestion in the periphery of the molecule. The steric congestion also appears to have affected bond parameters in the core. However, the coordination group bond parameters are that expected for a high-spin iron(III) porphyrinate with FeN(ave)=2.065 A, FeO= 1.752(1) A and a FeOFe angle of 178.6(6)°. The iron(III) atom is displaced 0.53 A from the quasi D2d ruffled core. Crystal data: monoclinic, a= 22.593(10), b=15.138(5), c=20.368(6) A and β=103.04(2)°, Z=4, space group C2/c, 4199 observed data collected on an automated four- circle diffractometer.

Excess energy dependence of internal conversion in the S1 state of azulene

The internal conversion (IC) rates for nine single vibronic levels of the S1 state of azulene, having from 0 to 2417 cm−1 of vibrational energy, have been measured by direct absorption spectroscopy in a pulsed, planar supersonic jet. The value of kIC at the vibrationless level is 9.4×1011 s−1 , with an approximately linear increase to 3.3×1012 s−1 at the highest level studied. This excess energy dependence of the rate constant is contrasted to that for S2 →S1 IC across an almost identical electronic energy gap, and an explanation involving low‐frequency modes is offered.

Van der Waals complexes of iodine with hydrogen and deuterium: Intermolecular potentials and laser‐induced photodissociation studies

A supersonic free jet expansion has been used to produce the van der Waals complexes of I2 with ortho‐ and para‐H2 and D2. The results of fluorescence excitation and dispersed emission spectroscopy were combined to deduce the parameters of the isotropic part of the intermolecular potential. Using a Morse potential these were found to be De=122 cm−1 and ωe=106 cm−1. Observations were made which also indicate the presence of anisotropy in the potential: different absorption frequencies for complexes containing ortho and para forms of H2 and D2, and different product state distributions when ortho and para H2I2 vibrationally predissociate. Approximate measurements of vibrational predissociation lifetimes of I2H2 and some I2–rare gas complexes are reported here, and their dependence on various molecular and intermolecular potential parameters is discussed in light of current theoretical treatments, namely,the energy gap law of Beswick and Jortner and the momentum gap law of Ewing. In the case of o‐H2I2, the r...