Magnetic Circular Dichroism of the CH Radical in an Argon Matrix

Abstract

Magnetic circular dichroism (MCD) and absorption spectra are reported for the CH (methylidyne or methyne) free radical isolated in an Ar matrix (CH/Ar) over the temperature range 1.4-15.9 K and the magnetic-field range 0-4 T. Data for the A 2 ∆ r X 2 Π, B 2 Σr X 2 Π, and C 2 Σ + r X 2 Π band systems indicate a substantial quenching of the orbital angular momentum of the CH ground-state term. This is interpreted as being a consequence of crystal-field interactions involving atoms of the host lattice. Analysis of the zeroth moments of the spectra yields an empirical spin-orbit coupling constant A Π ) 21 ( 1 cm -1 and orbital reduction factor κ ) 0.26 ( 0.03, corresponding to a crystal-field splitting of V Π ) 78 ( 14 cm -1 for the X 2 Π term. For the A 2 ∆ excited-state term, the first MCD moments give A ∆ ) 5 ( 1 cm -1 , and an upper limit of V ∆ j 15 cm -1 is estimated from the dispersion of the MCD.

Magnetic Circular Dichroism and Absorption Spectra of the NH Radical in an Argon Matrix

“…In a solid matrix, the A 3 Π term is also potentially susceptible to lowering of the orbital degeneracy due to crystal-fields (CF) interactions with nearby host atoms. The existence of such effects has been demonstrated previously for diatomic radicals with orbitally degenerate ground states, 1,2,30 for which the magnitude of the CF could be determined by moment analysis. However, lower moments (e.g., M 1 and A 0 ) obtained by integrating over the entire band system are invariant to unitary transformations on the excited-state basis; 3 hence the data in Figures 5-8 provide no information concerning CF splitting of the A 3 Π term of NH(D)/Ar.…”

“…This paper is the third in a series examining the absorption and magnetic circular dichroism (MCD) spectra of monohydride radicals of first-row elements isolated in Ar matrixes (XH/Ar). The earlier papers concerned OH 1 and CH, 2 which have orbitally degenerate 2 Π ground states and give MCD spectra dominated by zeroth moment Band C-term contributions. 3 The work presented here concerns the A 3 Π r X 3 Σtransition of the NH (imidogen or imine) and ND radicals in Ar.…”

“…The aims of this work are to (i) verify the MCD temperature dependence observed by Rose 22 for the A 3 Π r X 3 Σsystem of NH/Ar, (ii) provide a physically realistic model that accounts for the temperature and magnetic-field dependencies of the MCD, and (iii) compare and contrast the results and conclusions for NH/Ar with those obtained earlier for OH/Ar 1,23 and CH/ Ar. 2,23 II. Experimental Section MCD (∆A) and double-beam absorption (A) spectra were measured simultaneously using a spectrometer described elsewhere.…”

“…A similar problem was encountered in investigations of CH/Ar. 2,23 Partial analysis of the spectra is achieved below by using the method of moments. 3 The nth absorption (A n ) and MCD (M n ) moments are defined by Here E is the photon energy, E h is the absorption band barycenter (defined by A 1 ) 0), and the integrals are carried over the full envelope of the transition.…”

An accurate description of the ground and excited states of CH

“…149 Spectra of CH in a low temperature matrix have recently been obtained. 152,153 Theoretically, first-row hydrides were the main targets of early molecular computations beyond H 2 and H 2 ϩ . One reason for this was the relative abundance of early spectroscopic data for these molecules ͑and their cations͒ thus furnishing equilibrium separations, spectroscopic constants, and ground state symmetries.…”