Photoelectron Spectroscopy and Theoretical Studies of PCSe , AsCS , AsCSe , and NCSe : Insights into the Electronic Structures of the Whole Family of ECX Anions (E=N, P, As; X=O, S, Se).

The newly synthesized phosphorus- and arsenic-containing analogues of the thio- and seleno-cyanate anions, PCSe , AsCS , and AsCSe , as well as the known ion NCSe were investigated in the gas phase by negative-ion photoelectron spectroscopy (NIPES), velocity-map imaging (VMI) spectroscopy, and quantum-chemical computations. The electron affinities (EA), spin-orbit (SO) splittings, and "symmetric"/"asymmetric" stretching frequencies of the neutral radicals ECX (E=N, P, As; X=S, Se), generated by electron detachment from the corresponding anions, were obtained from the spectra. The calculated EAs, SO splittings, and vibrational frequencies are in excellent agreement with the experimental measurements.

Negative Ion Photoelectron Spectroscopy Confirms the Prediction of a Singlet Ground State for the 1,8-Naphthoquinone Diradical.

Negative ion photoelectron (NIPE) spectra, with 193, 266, 300, and 355 nm photons, of the radical anion of 1,8-naphthoquinone (1,8-NQ) have been obtained at 20 K. The electron affinity of 1,8-NQ is determined from the first resolved peak in the NIPE spectrum to be 2.965 ± 0.

Calculations on 1,8-naphthoquinone predict that the ground state of this diradical is a singlet.

(12/12)CASPT2, (16/14)CASPT2, B3LYP, and CCSD(T) calculations have been carried out on 1,8-Naphthoquinone (1,8-NQ), to predict the low-lying electronic states and their relative energies in this non-Kekulé quinone diradical. CASPT2 predicts a A ground state, with three other electronic states- B , B , and B -within about 10 kcal/mol of the ground state in energy. On the basis of the results of these calculations, it is predicted that NIPES experiments on 1,8-NQ will find that 1,8-NQ is a diradical with a singlet ground state.

Negative Ion Photoelectron Spectroscopy Confirms the Prediction of the Relative Energies of the Low-Lying Electronic States of 2,7-Naphthoquinone.

Cryogenic negative ion photoelectron (NIPE) spectra of the radical anion of 2,7-naphthoquinone (NQ) have been taken at 20 K, using 193, 240, 266, 300, and 355 nm lasers for electron detachment. The electron affinity of the NQ diradical is determined from the first resolved peak in the NIPE spectrum to be 2.880 ± 0.

Experimental and Theoretical Studies of the F + H-F Transition-State Region by Photodetachment of [F-H-F].

The transition-state (TS) region of the simplest heavy-light-heavy type of reaction, F + H-F → F-H + F, is investigated in this work by a joint experimental and theoretical approach. Photodetaching the bifluoride anion, [F···H···F], generates a negative ion photoelectron (NIPE) spectrum with three partially resolved bands in the electron binding energy (eBE) range of 5.4-7.

The Cope Rearrangement of 1,5-Dimethylsemibullvalene-2(4)-d : Experimental Evidence for Heavy-Atom Tunneling.

As an experimental test of the theoretical prediction that heavy-atom tunneling is involved in the degenerate Cope rearrangement of semibullvalenes at cryogenic temperatures, monodeuterated 1,5-dimethylsemibullvalene isotopomers were prepared and investigated by IR spectroscopy using the matrix isolation technique. As predicted, the less thermodynamically stable isotopomer rearranges at cryogenic temperatures in the dark to the more stable one, while broadband IR irradiation above 2000 cm results in an equilibration of the isotopomeric ratio. Since this reaction proceeds with a rate constant in the order of 10  s despite an experimental barrier of E =4.

Isotope-Controlled Selectivity by Quantum Tunneling: Hydrogen Migration versus Ring Expansion in Cyclopropylmethylcarbenes.

Using the tunneling-controlled reactivity of cyclopropylmethylcarbene, we demonstrate the viability of isotope-controlled selectivity (ICS), a novel control element of chemical reactivity where a molecular system with two conceivable products of tunneling exclusively produces one or the other, depending only on isotopic composition. Our multidimensional small-curvature tunneling (SCT) computations indicate that, under cryogenic conditions, 1-methoxycyclopropylmethylcarbene shows rapid H-migration to 1-methoxy-1-vinylcyclopropane, whereas deuterium-substituted 1-methoxycyclopropyl-d-methylcarbene undergoes ring expansion to 1-d-methylcyclobutene. This predicted change in reactivity constitutes the first example of a kinetic isotope effect that discriminates between the formation of two products.

Dioxygen: What Makes This Triplet Diradical Kinetically Persistent?

Experimental heats of formation and enthalpies obtained from G4 calculations both find that the resonance stabilization of the two unpaired electrons in triplet O, relative to the unpaired electrons in two hydroxyl radicals, amounts to 100 kcal/mol. The origin of this huge stabilization energy is described within the contexts of both molecular orbital (MO) and valence-bond (VB) theory. Although O is a triplet diradical, the thermodynamic unfavorability of both its hydrogen atom abstraction and oligomerization reactions can be attributed to its very large resonance stabilization energy.

Spectroscopic Characterization, Computational Investigation, and Comparisons of ECX (E = As, P, and N; X = S and O) Anions.

Three newly synthesized [Na(221-Kryptofix)] salts containing AsCO, PCO, and PCS anions were successfully electrosprayed into a vacuum, and these three ECX anions were investigated by negative ion photoelectron spectroscopy (NIPES) along with high-resolution photoelectron imaging spectroscopy. For each ECX anion, a well-resolved NIPE spectrum was obtained, in which every major peak is split into a doublet. The splittings are attributed to spin-orbit coupling (SOC) in the ECX radicals.

Why Are Addition Reactions to N Thermodynamically Unfavorable?

Thermochemical data are used to show that, of the 89.9 kcal/mol difference between the endothermicity of H addition to N (ΔH = 47.9 kcal/mol) and the exothermicity of H addition to acetylene (ΔH = -42.

Variations in Rotational Barriers of Allyl and Benzyl Cations, Anions, and Radicals.

High accuracy quantum chemical calculations show that the barriers to rotation of a CH group in the allyl cation, radical, and anion are 33, 14, and 21 kcal/mol, respectively. The benzyl cation, radical, and anion have barriers of 45, 11, and 24 kcal/mol, respectively. These barrier heights are related to the magnitude of the delocalization stabilization of each fully conjugated system.

A Joint Experimental and Computational Study of the Negative Ion Photoelectron Spectroscopy of the 1-Phospha-2,3,4-triazolate Anion, HCPN3(.).

We report here the results of a combined experimental and computational study of the negative ion photoelectron spectroscopy (NIPES) of the recently synthesized, planar, aromatic, HCPN3(-) ion. The adiabatic electron detachment energy of HCPN3(-) (electron affinity of HCPN3(•)) was measured to be 3.555 ± 0.

Negative ion photoelectron spectroscopy of PN: electron affinity and electronic structures of PN˙.

We report here a negative ion photoelectron spectroscopy (NIPES) and study of the recently synthesized planar aromatic inorganic ion PN, to investigate the electronic structures of PN and its neutral PN˙ radical. The adiabatic detachment energy of PN (electron affinity of PN˙) was determined to be 3.765 ± 0.

Negative ion photoelectron spectroscopy confirms the prediction that carbon trioxide (CO) has a singlet ground state.

The CO radical anion (CO˙) has been formed by electrospraying carbonate dianion (CO) into the gas phase. The negative ion photoelectron (NIPE) spectrum of CO˙ shows that, unlike the isoelectronic trimethylenemethane [C(CH)], carbon trioxide (CO) has a singlet ground state. From the NIPE spectrum, the electron affinity of singlet CO was, for the first time, directly determined to be EA = 4.

Theoretical Analysis of the Fragmentation of (CO)5: A Symmetry-Allowed Highly Exothermic Reaction that Follows a Stepwise Pathway.

B3LYP and CCSD(T) calculations, using an aug-cc-pVTZ basis set, have been carried out on the fragmentation of 1,2,3,4,5-cyclopentanepentone, (CO)(5), to five molecules of CO. Although this reaction is calculated to be highly exothermic and is allowed to be concerted by the Woodward-Hoffmann rules, our calculations find that the D(5h) energy maximum is a multidimensional hilltop on the potential energy surface. This D(5h) hilltop is 16-20 kcal/mol higher in energy than a C(2) transition structure for the endothermic cleavage of (CO)(5) to (CO)(4) + CO and 11-15 kcal/mol higher than a C(s) transition structure for the loss of two CO molecules.

Formation of Ground State Triplet Diradicals from Annulated Rosarin Derivatives by Triprotonation.

Annulated rosarins, β,β'-bridged hexaphyrin(1.0.1.

Negative Ion Photoelectron Spectroscopy Confirms the Prediction that 1,2,4,5-Tetraoxatetramethylenebenzene Has a Singlet Ground State.

The negative ion photoelectron (NIPE) spectrum of 1,2,4,5-tetraoxatetramethylenebenzene radical anion (TOTMB(•-)) shows that, like the hydrocarbon, 1,2,4,5-tetramethylenebenzene (TMB), the TOTMB diradical has a singlet ground state and thus violates Hund's rule. The NIPE spectrum of TOTMB(•-) gives a value of -ΔEST = 3.5 ± 0.

Nucleus-independent chemical shift analysis of the electronic states of the (CO)4, (CS)4, and (CSe)4 molecules.

NICS(1) calculations have been performed on the 8π and 10π singlet states and the 9π triplet state of (CO)4, (CS)4, and (CSe)4. The results show that transfer of electrons from the b2g σ MO into the a2u π MO decreases the NICS(1) value, indicating an increase in the diamagnetic ring current. The decreases in the calculated NICS(1) values are substantially larger in (CO)4 than in (CS)4 or (CSe)4.

The negative ion photoelectron spectrum of cyclopropane-1,2,3-trione radical anion, (CO)3(•-)--a joint experimental and computational study.

Negative ion photoelectron (NIPE) spectra of the radical anion of cyclopropane-1,2,3-trione, (CO)3(•-), have been obtained at 20 K, using both 355 and 266 nm lasers for electron photodetachment. The spectra show broadened bands, due to the short lifetimes of both the singlet and triplet states of neutral (CO)3 and, to a lesser extent, to the vibrational progressions that accompany the photodetachment process. The smaller intensity of the band with the lower electron binding energy suggests that the singlet is the ground state of (CO)3.

Computational and ¹³C investigations of the diazadienes and oxazadienes formed via the rearrangement of methylenecyclopropyl hydrazones and oximes.

Computational and further experimental investigations of the previously reported diazadienes, obtained via the rearrangement of methylenecyclopropyl hydrazone 1 are reported. Calculations at the CCSD(T)/cc-pVTZ//B3LYP/6-31G(d) level of theory indicate that the initially reported product 3 would, if formed, undergo rapid electrocyclic ring opening and, hence, would be unstable under the reaction conditions. Based on this computational prediction, further analysis of the (13)C NMR spectrum, previously attributed to 3, led to the revision of structure 3 to that of its N-tosylaminopyrrole constitutional isomer 11.

Heavy-atom tunneling in the ring opening of a strained cyclopropene at very low temperatures.

The highly strained 1H-bicyclo[3.1.0]-hexa-3,5-dien-2-one 1 is metastable, and rearranges to 4-oxacyclohexa-2,5-dienylidene 2 in inert gas matrices (neon, argon, krypton, xenon, and nitrogen) at temperatures as low as 3 K.

The negative ion photoelectron spectrum of meta-benzoquinone radical anion (MBQ˙⁻): a joint experimental and computational study.

Negative ion photoelectron (NIPE) spectra of the radical anion of meta-benzoquinone (MBQ, m-OC6H4O) have been obtained at 20 K, using both 355 and 266 nm lasers for electron photodetachment. The spectra show well-resolved peaks and complex spectral patterns. The electron affinity of MBQ is determined from the first resolved peak to be 2.

Why is (SiO)(4) calculated to be tetrahedral, whereas (CO)(4) is square planar? A molecular orbital analysis.

Qualitative molecular orbital (MO) theory predicts that square-planar tetrasilacyclobutanetetraone D4h-(SiO)4 should, like D4h-(CO)4, have a triplet ground state, and the results of the (U)CCSD(T)-F12b/cc-pVTZ-F12//(U)B3LYP/6-311+G(2df) calculations, reported here, confirm this expectation. Calculations at the same level of theory find that square-planar tetrasilacyclobutanetetrathione D4h-(SiS)4 also has a triplet ground state. However, these ab initio calculations predict that (SiO)4 and (SiS)4 both have a singlet state of much lower energy, with a tetrahedral (Td) equilibrium geometry and six, electron-deficient, Si-Si bonds.

Calculations on tunneling in the reactions of noradamantyl carbenes.

Noradamantylchlorocarbene has been found experimentally to undergo ring expansion to 2-chloroadamantene at cryogenic temperatures. The rate constant, calculated with inclusion of small-curvature tunneling, is within a factor of 2 of the rate constant measured at 9 K in a nitrogen matrix. Our calculations predict that noradamantylfluorocarbene will not be found to rearrange under these conditions.