High-Resolution Electronic Spectroscopy of the Doorway States to Intramolecular Charge Transfer

Reported here are several of the ground, first, and second excited state structures and dipole moments of three benchmark intramolecular charge transfer (ICT) systems; 4-(1<i>H</i>-pyrrol-1-yl)­benzonitrile (PBN), 4,4′-dimethylaminobenzonitrile (DMABN), and 4-(1-pyrrolidinyl)­benzonitrile (PYRBN), isolated in the gas phase and probed by rotationally resolved spectroscopy in a molecular beam. The related molecules 1-phenylpyrrole (PP) and 4-aminobenzonitrile (ABN) also are discussed. We find that the S₁ electronic state is of B symmetry in all five molecules. In PBN, a second excited state (S₂) of A symmetry is found only ∼400 cm^–1 above the presumed origin of the S₁ state. The change in dipole moment upon excitation to the A state is measured to be Δμ ≈ 3.0 D, significantly smaller than the value predicted by theory and also smaller than that observed for the “anomalous” ICT band of PBN in solution. The B state dipole moments of DMABN and PYRBN are large, ∼10.6 D, slightly larger than those attributed to “normal” LE fluorescence in solution. In addition, we find the unsaturated donor molecules (PP, PBN) to be twisted in their ground states and to become more planar upon excitation, even in the A state, whereas the saturated donor molecules (ABN, DMABN, PYRBN), initially planar, either remain planar or become more twisted in their excited states. It thus appears that the model that is appropriate for describing ICT in these systems depends on the geometry of the ground state

An Isolated Complex of Ethyne and Gold Iodide Characterized by Broadband Rotational Spectroscopy and Ab initio Calculations

A molecular complex of C₂H₂ and AuI has been generated and isolated in the gas phase through laser ablation of a gold surface in the presence of an expanding sample containing small percentages of C₂H₂ and CF₃I in a buffer gas of argon. Rotational, <i>B</i>₀, centrifugal distortion, Δ_<i>J</i> and Δ_<i>JK</i>, and nuclear quadrupole coupling constants, χ_<i>aa</i>(Au), χ_<i>bb</i>(Au) – χ_<i>cc</i>(Au), χ_<i>aa</i>(I), and χ_<i>bb</i>(I) – χ_<i>cc</i>(I), are measured for three isotopologues of C₂H₂···AuI through broadband rotational spectroscopy. The complex is <i>C</i>_2<i>v</i> and T-shaped with C₂H₂ coordinating to the gold atom via donation of electrons from the π-orbitals of ethyne. On formation of the complex, the CC bond of ethyne extends by 0.032(4) Å relative to <i>r</i>(CC) in isolated ethyne when the respective <i>r</i>₀ geometries are compared. The geometry of ethyne distorts such that ∠(*CH) (where * indicates the midpoint of the CC bond) is 194.7(12)° in the <i>r</i>₀ geometry of C₂H₂···AuI. <i>Ab initio</i> calculations at the CCSD­(T)­(F12*)/AVTZ level are consistent with the experimentally determined geometry and further allow calculation of the dissociation energy (<i>D</i>_e) as 136 kJ mol^–1. The χ_<i>aa</i>(Au) and χ_<i>aa</i>(I) nuclear quadrupole coupling constants of AuI and also the AuI bond length change significantly on formation of the complex consistent with the strong interaction calculated to occur between C₂H₂ and AuI

An Isolated Complex of Ethyne and Gold Iodide Characterized by Broadband Rotational Spectroscopy and Ab initio Calculations

A molecular complex of C₂H₂ and AuI has been generated and isolated in the gas phase through laser ablation of a gold surface in the presence of an expanding sample containing small percentages of C₂H₂ and CF₃I in a buffer gas of argon. Rotational, <i>B</i>₀, centrifugal distortion, Δ_<i>J</i> and Δ_<i>JK</i>, and nuclear quadrupole coupling constants, χ_<i>aa</i>(Au), χ_<i>bb</i>(Au) – χ_<i>cc</i>(Au), χ_<i>aa</i>(I), and χ_<i>bb</i>(I) – χ_<i>cc</i>(I), are measured for three isotopologues of C₂H₂···AuI through broadband rotational spectroscopy. The complex is <i>C</i>_2<i>v</i> and T-shaped with C₂H₂ coordinating to the gold atom via donation of electrons from the π-orbitals of ethyne. On formation of the complex, the CC bond of ethyne extends by 0.032(4) Å relative to <i>r</i>(CC) in isolated ethyne when the respective <i>r</i>₀ geometries are compared. The geometry of ethyne distorts such that ∠(*CH) (where * indicates the midpoint of the CC bond) is 194.7(12)° in the <i>r</i>₀ geometry of C₂H₂···AuI. <i>Ab initio</i> calculations at the CCSD­(T)­(F12*)/AVTZ level are consistent with the experimentally determined geometry and further allow calculation of the dissociation energy (<i>D</i>_e) as 136 kJ mol^–1. The χ_<i>aa</i>(Au) and χ_<i>aa</i>(I) nuclear quadrupole coupling constants of AuI and also the AuI bond length change significantly on formation of the complex consistent with the strong interaction calculated to occur between C₂H₂ and AuI