Scott W. Wren

The photoelectron spectrum of CCl2-: the convergence of theory and experiment after a decade of debate.

We report new 351 nm negative ion photoelectron spectra of CCl(2)(-), CBr(2)(-), and CI(2)(-). This study was undertaken in an attempt to understand the major discrepancy between dihalocarbene (CX(2), X = Cl, Br, I) singlet-triplet splittings reported by our laboratory (R. L. Schwartz, G. E. Davico, T. M. Ramond, W. C. Lineberger, J. Phys. Chem. A., 1999, 103, 8213) and new theoretical values. Our recent experiments show that a dihalomethyl anion (CHX(2)(-)) contaminant in the dihalocarbene anion beam, previously considered insignificant, made a major contribution to the reported CX(2)(-) photoelectron spectra. Thus, the interpretations of the earlier CX(2)(-) spectra and the reported singlet-triplet splittings were incorrect. Replacing O(-) with OH(-) in the anion formation process yields a pure dihalomethyl anion, whose highly structured photoelectron spectrum can be subtracted from the contaminated spectrum, yielding a clean CX(2)(-) photoelectron spectrum. The new CCl(2)(-) photoelectron spectrum displays resolved vibronic transitions to the two lowest electronic states of CCl(2): X(1)A(1) and a(3)B(1). The electron affinity of X(1)A(1) CCl(2) is 1.593(6) eV. A large change in geometry between the anion and the neutral triplet state precludes the direct observation of the triplet origin. The energy difference between the X(1)A(1) and a(3)B(1) states of CCl(2) is estimated to be approximately 0.9(2) eV, consistent with high-level theoretical studies. While we confirm similar dihalomethyl anion contaminants in the earlier photoelectron spectra of CBr(2)(-) and CI(2)(-) and report new photoelectron spectra for these ions, the paucity of resolved features in the spectra provides limited additional thermochemical information.

C-H bond strengths and acidities in aromatic systems: effects of nitrogen incorporation in mono-, di-, and triazines.

The negative ion chemistry of five azine molecules has been investigated using the combined experimental techniques of negative ion photoelectron spectroscopy to obtain electron affinities (EA) and tandem flowing afterglow-selected ion tube (FA-SIFT) mass spectrometry to obtain deprotonation enthalpies (Δ(acid)H(298)). The measured Δ(acid)H(298) for the most acidic site of each azine species is combined with the EA of the corresponding radical in a thermochemical cycle to determine the corresponding C-H bond dissociation energy (BDE). The site-specific C-H BDE values of pyridine, 1,2-diazine, 1,3-diazine, 1,4-diazine, and 1,3,5-triazine are 110.4 ± 2.0, 111.3 ± 0.7, 113.4 ± 0.7, 107.5 ± 0.4, and 107.8 ± 0.7 kcal mol(-1), respectively. The application of complementary experimental methods, along with quantum chemical calculations, to a series of nitrogen-substituted azines sheds light on the influence of nitrogen atom substitution on the strength of C-H bonds in six-membered rings.

Ground and low-lying excited states of propadienylidene (H2C=C=C:) obtained by negative ion photoelectron spectroscopy

A joint experimental-theoretical study has been carried out on electronic states of propadienylidene (H(2)CCC), using results from negative-ion photoelectron spectroscopy. In addition to the previously characterized X(1)A(1) electronic state, spectroscopic features are observed that belong to five additional states: the low-lying ã(3)B(1) and b(3)A(2) states, as well as two excited singlets, Ã(1)A(2) and B(1)B(1), and a higher-lying triplet, c(3)A(1). Term energies (T(0), in cm(-1)) for the excited states obtained from the data are: 10,354±11 (ã(3)B(1)); 11,950±30 (b(3)A(2)); 20,943±11 (c(3)A(1)); and 13,677±11 (Ã(1)A(2)). Strong vibronic coupling affects the Ã(1)A(2) and B(1)B(1) states as well as ã(3)B(1) and b(3)A(2) and has profound effects on the spectrum. As a result, only a weak, broadened band is observed in the energy region where the origin of the B(1)B(1) state is expected. The assignments here are supported by high-level coupled-cluster calculations and spectral simulations based on a vibronic coupling Hamiltonian. A result of astrophysical interest is that the present study supports the idea that a broad absorption band found at 5450 Å by cavity ringdown spectroscopy (and coincident with a diffuse interstellar band) is carried by the B(1)B(1) state of H(2)CCC.

Electronic states of the quasilinear molecule propargylene (HCCCH) from negative ion photoelectron spectroscopy

We use gas-phase negative ion photoelectron spectroscopy to study the quasilinear carbene propargylene, HCCCH, and its isotopologue DCCCD. Photodetachment from HCCCH– affords the X̃(3B) ground state of HCCCH and its ã(1A), b̃ (1B), d̃(1A2), and B̃(3A2) excited states. Extended, negatively anharmonic vibrational progressions in the X̃(3B) ground state and the open-shell singlet b̃ (1B) state arise from the change in geometry between the anion and the neutral states and complicate the assignment of the origin peak. The geometry change arising from electron photodetachment results in excitation of the ν4 symmetric CCH bending mode, with a measured fundamental frequency of 363 ± 57 cm(–1) in the X̃(3B) state. Our calculated harmonic frequency for this mode is 359 cm(–1). The Franck–Condon envelope of this progression cannot be reproduced within the harmonic approximation. The spectra of the ã(1A), d̃(1A2), and B̃(3A2) states are each characterized by a short vibrational progression and a prominent origin peak, establishing that the geometries of the anion and these neutral states are similar. Through comparison of the HCCCH– and DCCCD– photoelectron spectra, we measure the electron affinity of HCCCH to be 1.156 ± (0.095)(0.010) eV, with a singlet–triplet splitting between the X̃(3B) and the ã(1A) states of ΔEST = 0.500 ± (0.01)(0.10) eV (11.5 ± (0.2)(2.3) kcal/mol). Experimental term energies of the higher excited states are T0 [b̃(1B)] = 0.94 ± (0.20)(0.22) eV, T0 [d̃(1A2)] = 3.30 ± (0.02)(0.10) eV, T0 [B̃(3A2)] = 3.58 ± (0.02)(0.10) eV. The photoelectron angular distributions show significant π character in all the frontier molecular orbitals, with additional σ character in orbitals that create the X̃(3B) and b̃(1B) states upon electron detachment. These results are consistent with a quasilinear, nonplanar, doubly allylic structure of X̃(3B) HCCCH with both diradical and carbene character.

Photoelectron spectra of dihalomethyl anions: Testing the limits of normal mode analysis

We report the 364-nm negative ion photoelectron spectra of CHX(2)(-) and CDX(2)(-), where X = Cl, Br, and I. The pyramidal dihalomethyl anions undergo a large geometry change upon electron photodetachment to become nearly planar, resulting in multiple extended vibrational progressions in the photoelectron spectra. The normal mode analysis that successfully models photoelectron spectra when geometry changes are modest is unable to reproduce qualitatively the experimental data using physically reasonable parameters. Specifically, the harmonic normal mode analysis using Cartesian displacement coordinates results in much more C-H stretch excitation than is observed, leading to a simulated photoelectron spectrum that is much broader than that which is seen experimentally. A (2 + 1)-dimensional anharmonic coupled-mode analysis much better reproduces the observed vibrational structure. We obtain an estimate of the adiabatic electron affinity of each dihalomethyl radical studied. The electron affinity of CHCl(2) and CDCl(2) is 1.3(2) eV, of CHBr(2) and CDBr(2) is 1.9(2) eV, and of CHI(2) and CDI(2) is 1.9(2) eV. Analysis of the experimental spectra illustrates the limits of the conventional normal mode approach and shows the type of analysis required for substantial geometry changes when multiple modes are active upon photodetachment.

Photoelectron Spectroscopy of Anilinide and Acidity of Aniline

The photoelectron spectrum of the anilinide ion has been measured. The spectrum exhibits a vibrational progression of the CCC in-plane bending mode of the anilino radical in its electronic ground state. The observed fundamental frequency is 524 ± 10 cm(-1). The electron affinity (EA) of the radical is determined to be 1.607 ± 0.004 eV. The EA value is combined with the N-H bond dissociation energy of aniline in a negative ion thermochemical cycle to derive the deprotonation enthalpy of aniline at 0 K; Δ(acid)H(0)(PhHN-H) = 1535.4 ± 0.7 kJ mol(-1). Temperature corrections are made to obtain the corresponding value at 298 K and the gas-phase acidity; Δ(acid)H(298)(PhHN-H) = 1540.8 ± 1.0 kJ mol(-1) and Δ(acid)G(298)(PhHN-H) = 1509.2 ± 1.5 kJ mol(-1), respectively. The compatibility of this value in the acidity scale that is currently available is examined by utilizing the acidity of acetaldehyde as a reference.

Ion Chemistry of 1H-1,2,3-Triazole. 2. Photoelectron Spectrum of the Iminodiazomethyl Anion and Collision Induced Dissociation of the 1,2,3-Triazolide Ion

The 363.8 nm photoelectron spectrum of the iminodiazomethyl anion has been measured. The anion is synthesized through the reaction of the hydroxide ion (HO-) with 1 H-1,2,3-triazole in helium buffer gas in a flowing afterglow ion source. The observed spectrum exhibits well-resolved vibronic structure of the iminodiazomethyl radical. Electronic structure calculations have been performed at the B3LYP/6-311++G(d,p) level of theory to study the molecular structure of the ion. Equilibrium geometries of four possible conformers of the iminodiazomethyl anion have been obtained from the calculations. Spectral simulations have been performed on the basis of the calculated geometries and normal modes of these conformationally isomeric ions and the corresponding radicals. The spectral analysis suggests that the ions of two conformations are primarily formed in the aforementioned reaction. The relative abundance of the two conformers substantially deviates from the thermal equilibrium populations, and it reflects the potential energy surfaces relevant to conformational isomerization processes. The electron affinities of the ( ZE)- and ( EE)-iminodiazomethyl radicals have been determined to be 2.484 +/- 0.007 and 2.460 +/- 0.007 eV, respectively. The energetics of the iminodiazomethyl anion is compared with that of the most stable structural isomer, the 1,2,3-triazolide ion. Collision-induced dissociation of the 1,2,3-triazolide ion has also been studied in flowing afterglow-selected ion flow tube experiments. Facile fragmentation generating a product ion of m/ z 40 has been observed. DFT calculations suggest that fragmentation of the 1,2,3-triazolide ion to the cyanomethyl anion and N2 is exothermic. The stability of the ion is discussed in comparison with other azolide ions with different numbers of N atoms in the five-membered ring.