Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY

Spectroscopic and Theoretical Aspects

BY D. PHILLIPS

Introduction

The format here is the same as that used in Volume 6 of the series. Some special emphasis has been given to interesting developments in simultaneous two-photon absorption and excitation spectroscopy and photofragment spectroscopy, but otherwise reportage in this section has been kept to a minimum.

2 Calculations

As before, this section is confined to estimates of energy levels, transition energies, and oscillator strengths obtained through calculation. Papers dealing with potential-energy surfaces in relation to photochemical reaction and non-radiative decay are found in later sections.

A brief account has been given of the applications of qualitative MO theory, and the use of a least-squares method in CI calculations has been discussed. A new vector method has been proposed to describe the electronic states of atoms and molecules, and the use of one-electron MO theory for energy-level determination discussed. Ab initio computations of spin-orbit interactions in polyatomic molecules using gaussian orbitals and optimal orbitals for SCF CI calculations on excited states have been considered. Polynomial expansion methods for energies and excitation strengths have been outlined. Two new methods, the multiconfiguration electron-hole potential method and a time-dependent variation perturbation approach, have been used to calculate electronic transition parameters in a variety of molecules.

The generally accepted basis for the widely used Hund's rules for predicting the ordering of electronic states has been challenged in recent years, yet the rules appear to be valid. A reformulation of the rules in a strict SCF approximation in which many of the elements of the traditional theory are retained has been proposed, to surmount this difficulty. Electron repulsion in the singlet and triplet states of the helium atom, natural orbitals of several excited states of this atom, and the application of the SCF method to 1P and 3P excited states of two-electron atomic systems have been discussed recently. Among papers appearing concerned with the spectroscopy and energy levels of atomic species are included those on the subjects of Li (II), K+ (I), the Be (1s2s2 -> 1s22s2p) and B (1s22s22p -> 1s22s2p2) transitions, the C (2s2p3) 3S0 ->1D0 isoelectronic sequence, the vacuum-u.v. lines in N (I) to N (IV), the Na S states, oscillator strengths in elements of the iron group, isotope shifts in the arc spectrum of xenon, excitation energies in other atomic systems, and van der Waals interactions for atoms in excited states.

There have been several studies on diatomic species, ions in particular, which indicate the growing current interest in the experimental study of such molecular entities. Theoretical studies have been carried out on H2+, HD+, HeH+, Ne2+, N2+, Li2+, and other ions.

Neutral species investigated include H2, He2, Ne2, Ar2, LiH,> LiF, N2, NO, OH, BH, MgH2, Na2, C2, and CH4.

Triatomic species investigated are relatively few. Several papers have investigated the excited states of the water molecule using ab initio methods, including the improved virtual orbital and CI treatments. Good agreement with experiment was obtained in these studies. In one case it was shown that the 9.81 eV triplet state observed in electron-impact studies corresponds to the 10.17eV singlet (1A1[b1 -> 3px]) rather than the expected 10.00 eV singlet (1B1[b1 -> 3pz]) owing to the larger splitting between the and states of 0.46 eV compared with that between the 4B1 and 3B1 states of only 0.08 eV which rises from the magnitudes of the corresponding exchange integrals. It is of interest to compare these results with those obtained in a photo-fluorescence excitation spectroscopic study of H2O described in a later section. Rydberg states in superexcited states of H2O and NH3 and HF have been discussed.

The existence of a 3A" state of the HNO radical at 5485 cm-1 lying between the 1A" excited and 1A" ground states has been predicted from an equation-of-motion-method study. The usual assumption that electrons are further apart in triplet states than in singlet states has been challenged on the basis of calculations performed on the NH and CH2 species, in which it is shown that electron repulsion can be greater in the state of lowest energy for a number of systems, in accord with statements made earlier concerning Hund's rules.

Methylene (carbene) is a species of some photochemical interest, and has also been much studied from a theoretical standpoint. It possesses a triplet ground state, whereas diflu or omethylene is reported to have a singlet ground state, with CHF having a singlet ground state if electron correlation is included in computations, but a triplet ground state within the SCF approximation. Computed triplet-singlet energy separations including electron correlation were 10, 6, – 11, and – 47 kcal mol-1 for CH2, CHCH3, CHF, and CF2, respectively. The spectroscopy of CH2 requires that the first excited triplet state of CH2 be highly bent, 3A2 in character, and 8.75 eV above the ground state, whereas ab initio calculations show that the 3A2 state will decrease in energy with decreasing angle. Recent SCF calculations have shown that there are three 3A2 states which are highly bent, and in the right energy region (7.5 – 8.9 eV), but which result from 3a1 -> 3py, 3dxy, and 3dyz excitations rather than the 1b2 -> 3a1 so-called valence excitation previously considered. Spin-orbit contributions to the singlet-triplet splitting in methylene, ab initio methods for correlated wavefunctions of the ground and excited states of methylene, and the vertical ionization potential of CF2 have been discussed. CI studies on ozone and O3+ have been reported.

It has been shown recently that electronically excited states of CO2 which arise from the same electronic configuration can in fact have different degrees of Rydberg character; for example, together with the 1Σg ground state, the 3Σu+1,3Σu-, and 1,3Δu singly excited states are valence states, whereas the 1Σu+ is of Rydberg character. Recent calculations on acetylene have reinforced this point, although earlier assignments of the acetylene spectrum based upon the method used in this study have been challenged recently.

INDO calculations on the potential curves of ethylene give very poor agreement with ab initio results. Reports have appeared on the geometries of excited states of small polyenes, the low-lying electronic states of the ethynyl free radical, the vibronic structure of the π -> π* transition in cis-stilbene, the singlet-state geometries of diphenylacetylene, and the dissociation products of the HN2+ ion in its ground and excited states.

Formaldehyde, being the simplest aliphatic carbonyl compound, has been widely studied from a theoretical standpoint. These studies have been concerned variously with Rydberg transitions and the lower-lying n -> π* and π -> π* transitions, and include an excellent review by Moule and Walsh. A theoretical description has been given of the peroxyformyl radical, a species thought to be of importance in polluted urban atmospheres. Results indicate that the excited state of this radical may be formed by association of O2 and HCO in their ground states, and may decompose readily to give OH and CO2. INDO calculations on the 1A1, 1A", and 3A" and states of keten and diazomethane, 3A1, 1E, and 1A1 the and states of methylnitrene, ab initio calculations on the electronic spectrum of ethane, and excited electronic states of a-dicarbonyls, including biacetyl, have been reported. There have also been a number of reports concerned with the electronic excited states of radicals and radical ions.

Simple rules for an estimate of correlation effects in the low-lying states of alternant hydrocarbons have been formulated, and the polarizabilities of the three lowest singlet and triplet states of a number of conjugated molecules have been calculated using a perturbation theory approach, which also shows that methods based upon Hiickel theory give unsatisfactory results. SCF–CI calculations on the luminescent transitions from equilibrium excited states in aromatic molecules have also been reported. Ab initio, PPP-type, and equations-of-motion-type calculations on benzene, and other calculations concerned with spin density, vibronic interactions, and conjugation in excited electronic states in the same molecule, have been described.

A MINDO/3 study of the singlet and triplet forms of the isomeric bisdehydrobenzenes shows that the singlet state of (1) is the more stable, but that the singlet of (2) is at least comparable in stability, and such intermediates merit consideration in benzyne reaction mechanisms.

Calculation has shown that the 1B3u, 3B2u, and 3B1g and states of naphthalene are as polarizable as the ground state, whereas the and states are much more so, especially in the long-axis direction. The polarizability of the 1B1g state is extremely small. Several studies of the pyrene crystal have been reported. A valence-bond model for the lone-pair interactions in pyrazine, the electronic structure and spectra of quinalene, quinolines, and benzoquinolines, and calculated spectra in other azanaphthalenes and pyridine have been reported, and SCF–CI calculations on the excited-state geometries of p-fluorophenol and p-fluoroaniline carried out. Excited-state energies in p-benzoqinone, protonated aromatic carbonyl compounds, halogen-substituted benzaldehydes and benzoyl halides, and dibenzofuran, and the use of the CNDO method to describe triplet excited states of organic molecules have been the subjects of recent reports.

Calculations have been carried out on the spectroscopy and excited-state energies and geometries of polyenes, the 1,5-cyclo-octadiyne radical cation, the visual chromophore, 11-cis-retinal, polyene carbaldehydes, aromatic hydrocarbon anions and cations, trimethylenemethane, thiamine derivatives, porphyrins, lumiflavin, chlorophyll, nucleic acids, and ferrocene.

Many other papers have appeared concerned with the assignment of electronic transitions based upon various types of calculations, and these will be found in later sections of this chapter, in which experimental work is discussed.

3 Spectra

The format here is identical with that in Volume 5.

Absorption. — The studies reported here are conventional one-photon absorption studies on ground-state molecules and atoms. The use of a rapidly tuneable CW dye laser for direct absorption spectroscopy has been described. Transition probabilities in the spectra of Ne (I), and the classification of the 650.4 nm line of Xe have been discussed. Pressure-broadening coefficients for the atomic iodine 2P1/2 -2P3/2 transition for CO2, N2, He, Ne, Ar, Kr, and Xe have been measured as 7.4 [+ or -] 0.7, 6.2 [+ or -] 0.8, 3.6 [+ or -] 0.3, 4.3 [+ or -] 0.4, 5.1 [+ or -] 0.5, 4.4 [+ or -] 0.4, and 3.0 [+ or -] 0.3, respectively. Measurements of the polarizabilities of alkalimetal atoms have been described, and hyperfine interactions in the excited states of sodium discussed. Excitation parameters in the vacuum-u.v. region for metal atoms, including thallium, chromium, strontium, and copper, have been reported. The vacuum-u.v. absorption spectrum of mercury atoms in rare-gas matrices correlates with that of mercury vapour, but is shifted from it by as much as 2000 cm-1 for the 1S0 ->1P1 transition, and shows fine structure. The effect of H-Hg interactions on the line-shape of the 1S0 ->3P1 253.7 nm transition in mercury has also been discussed.

The fine structure of Rydberg, Lyman, and Werner bands of the H2 molecule, the high-resolution spectrum of the Xe2 molecule in the 115 — 130 nm region, and a comprehensive re-analysis of the O2([??]3Σu<- [??]Σg-) Schumann-Runge band system have been reported. The optical absorptions in solid O2 have been discussed from a theoretical standpoint. High-resolution studies have been carried out on the [??]1Σ+<- [??]Σ+ transitions of CO, the [??]2Π <- [??]2Π system in matrix-isolated NO, and the McLennan bands of I2. The dissociation energies of diatomic halogen fluorides and other molecules, and r-centroids and Franck-Condon factors in the [??]2<- [??]2ΣBeF system and in other molecules, a new band system in SiO([??]1<- [??]1Σ), the absorption spectrum of diatomic phosphorus between 137 and 60 nm, absorption and fluorescence of gaseous TeSe, optical spectra of matrix-isolated Be and Be2, and pressure effects on the vibronic transitions of NO and NH3 have been reported.

The bond length and angle of the excited 3B1 state of SO2 have been calculated to be 1.491 [+ or -] 0.002 Å and 126.1 [+ or -] 0.1° from a modified Franck-Condon analysis of the absorption and emission spectra. Photoabsorption of solid CO2 and rotational analysis of the 593.3 and 800 — 900 nm bands of NO2 have been reported. The application of the laser spectroscopy of supersonic molecular beams to the NO2 spectrum has been discussed. The [??]2Σ+<- [??]2Π band system of NCO, the vacuum-u.v. absorption spectrum of OCSe, and excited states of the NO+ and H2O+ ionic species have been discussed in recent papers. The vapour-phase absorption spectra of cyanogen, cyanoacetylene, dicyanoacetylene, and dicyanodiacetylene have been shown to be dominated by a 1Σu+<- 1Σg+ system arising from the lowest π <- π* configuration, together with several Rydberg bands. A moderately intense intra-valence-shell band assigned as a 1Πu ->1Σg+ transition arising from the lowest n -> π* configuration is also characteristic. The CN bond dissociation energy in nitrosyl cyanide has been discussed.