Effects of Ethynyl Substitution on Cyclobutadiene

The effects of ethynyl substitution on cyclobutadiene are explored via density functional theory and coupled-cluster calculations. The computed singlet–triplet gaps indicate a monotonic dependence on the degree of ethynyl substitution, which differentially stabilizes the triplet relative to the singlet ground state and reduces the gap. A series of isodesmic, homodesmotic, and hyperhomodesmotic equations are employed to quantify the stabilization upon ethynyl substitution. Analyses that rely on a simple isodesmic equation and/or B3LYP/6-31G(d) values are found to be problematic. Analyses that rely on homodesmotic or hyperhomodesmotic equations, in conjunction with CCSD/cc-pVDZ values, are more robust. Using a hyperhomodesmotic equation to assess the stabilization enthalpies of tetra-substituted singlet cyclobutadienes, our analysis predicts tetramethylcyclobutadiene (Δ<i>H</i>⁰_rxn = −17.3 kcal/mol) to be more stable than tetraethynylcyclobutadiene (Δ<i>H</i>⁰_rxn = −11.7 kcal/mol), which, in turn, is substantially more stable than tetracyanocyclobutadiene (Δ<i>H</i>⁰_rxn = +12.7 kcal/mol)

Photochemistry of CpMn(CO)₃ and Related Derivatives: Spectroscopic Observation of Singlet and Triplet CpMn(CO)₂

We report novel photochemistry derived from (η⁵-C₅H₅)­Mn­(CO)₃ (<b>1a</b>), (η⁵-C₅H₄Me)­Mn­(CO)₃ (<b>1b</b>), (η⁵-C₅Me₅)­Mn­(CO)₃ (<b>1c</b>), and (η⁵-indenyl)­Mn­(CO)₃ (<b>1d</b>). Photolysis (>261 nm, 1 h) of the parent tricarbonyl (<b>1a</b>–<b>d</b>), matrix isolated in argon at 10 K, yields two species: the expected singlet dicarbonyl ¹(η⁵-L)­Mn­(CO)₂ (^<b>1</b><b>2a</b>–<b>d</b>) and an additional compound assigned as the triplet dicarbonyl ³(η⁵-L)­Mn­(CO)₂ (^<b>3</b><b>2a</b>–<b>d</b>). Density functional theory calculations (B3LYP/LANL2DZ) support the structural assignments for ^<b>1</b><b>2</b> and ^<b>3</b><b>2</b>. Natural bond orbital population analyses of ^<b>1</b><b>2a</b> and ^<b>3</b><b>2a</b> explain the source of the large coupling (Δν_CO 153 cm^–1) between the carbonyl stretching vibrations in ^<b>3</b><b>2a</b>. The triplet isomer (^<b>3</b><b>2</b>) is metastable, even at temperatures as low as 10 K. We determined the rate constants for the thermal isomerization ^<b>3</b><b>2 </b>→ ^<b>1</b><b>2</b> using dispersive kinetic analysis. As revealed by these rate constants, the triplet complexes display the following order of stability in this system: Ind ≫ Cp ≈ Cp′ > Cp*. The spectroscopy and kinetics observed in various matrices (Ar, CH₄, and Xe) do not differ appreciably. Experimental and computational results suggest that the singlet–triplet energy gap (Δ<i>E</i>_ST) of CpMn­(CO)₂ (<b>2a</b>) must be smaller than previous estimates

Rotational Spectroscopy of 1‑Cyano-2-methylenecyclopropane (C₅H₅N)A Newly Synthesized Pyridine Isomer

The gas-phase rotational spectrum of 1-cyano-2-methylenecyclopropane (C1, C5H5N), an isomer of pyridine, is presented for the first time, covering the range from 235 to 500 GHz. Over 3600 a-, b-, and c-type transitions for the ground vibrational state have been assigned, measured, and least-squares fit to partial-octic A- and S-reduced distorted-rotor Hamiltonians with low statistical uncertainty (σfit = 42 kHz). Transitions for the two lowest-energy fundamental states (ν27 and ν26) and the lowest-energy overtone (2ν27) have been similarly measured, assigned, and least-squares fit to single-state Hamiltonians. Computed vibration–rotation interaction constants (B0–Bv) using the B3LYP and MP2 levels of theory are compared with the corresponding experimental values. Based upon our preliminary analysis, the next few vibrationally excited states form one or more complex polyads of interacting states via Coriolis and anharmonic coupling. The spectroscopic constants and transition frequencies presented here form the foundation for both future laboratory spectroscopy and astronomical searches for 1-cyano-2-methylenecyclopropane

Aryl Nitrene Rearrangements: Spectroscopic Observation of a Benzazirine and Its Ring Expansion to a Ketenimine by Heavy-Atom Tunneling

In the photodecompositions of 4-methoxyphenyl azide (<b>1</b>) and 4-methylthiophenyl azide (<b>5</b>) in argon matrixes at cryogenic temperatures, benzazirine intermediates were identified on the basis of IR spectra. As expected, the benzazirines photochemically rearranged to the corresponding ketenimines and triplet nitrenes. Interestingly, with the methylthio substituent, the rearrangement of benzazirine <b>8</b> to ketenimine <b>7</b> occurred at 1.49 × 10^–5 s^–1 even in the dark at 10 K, despite a computed activation barrier of 3.4 kcal mol^–1. Because this rate is 10⁵⁷ times higher than that calculated for passing over the barrier and because it shows no temperature dependence, the rearrangement mechanism is interpreted in terms of heavy-atom tunneling

Photochemistry of 2‑Formylphenylnitrene: A Doorway to Heavy-Atom Tunneling of a Benzazirine to a Cyclic Ketenimine

The slippery potential energy surface of aryl nitrenes has revealed unexpected and fascinating reactions. To explore such a challenging surface, one powerful approach is to use a combination of a cryogenic matrix environment and a tunable narrowband radiation source. In this way, we discovered the heavy-atom tunneling reaction involving spontaneous ring expansion of a fused-ring benzazirine into a seven-membered ring cyclic ketenimine. The benzazirine was generated in situ by the photochemistry of protium and deuterated triplet 2‑formylphenylnitrene isolated in an argon matrix. The ring-expansion reaction takes place at 10 K with a rate constant of ∼7.4 × 10^–7 s^–1, despite an estimated activation barrier of 7.5 kcal mol^–1. Moreover, it shows only a marginal increase in the rate upon increase of the absolute temperature by a factor of 2. Computed rate constants with and without tunneling confirm that the reaction can only occur by a tunneling process from the ground state at cryogenic conditions. It was also found that the ring-expansion reaction rate is more than 1 order of magnitude faster when the sample is exposed to broadband IR radiation

Photoisomerization of (Cyanomethylene)cyclopropane (C₅H₅N) to 1‑Cyano-2-methylenecyclopropane in an Argon Matrix

Broad-band ultraviolet photolysis (λ > 200 nm) of (cyanomethylene)cyclopropane (5) in an argon matrix at 20 K generates 1-cyano-2-methylenecyclopropane (7), a previously unknown compound. This product was initially identified by comparison of its infrared spectrum to that predicted by an anharmonic MP2/6-311+G(2d,p) calculation. This assignment was unambiguously confirmed by the synthesis of 1-cyano-2-methylenecyclopropane (7) and observation of its authentic infrared spectrum, which proved identical to that of the observed photoproduct. We investigated the singlet and triplet potential energy surfaces associated with this isomerization process using density functional theory and multireference calculations. The observed rearrangement of compound 5 to compound 7 is computed to be endothermic (3.3 kcal/mol). We were unable to observe the reverse reaction (7 → 5) under the photochemical conditions

Photoisomerization of (Cyanomethylene)cyclopropane (C₅H₅N) to 1‑Cyano-2-methylenecyclopropane in an Argon Matrix

Broad-band ultraviolet photolysis (λ > 200 nm) of (cyanomethylene)cyclopropane (5) in an argon matrix at 20 K generates 1-cyano-2-methylenecyclopropane (7), a previously unknown compound. This product was initially identified by comparison of its infrared spectrum to that predicted by an anharmonic MP2/6-311+G(2d,p) calculation. This assignment was unambiguously confirmed by the synthesis of 1-cyano-2-methylenecyclopropane (7) and observation of its authentic infrared spectrum, which proved identical to that of the observed photoproduct. We investigated the singlet and triplet potential energy surfaces associated with this isomerization process using density functional theory and multireference calculations. The observed rearrangement of compound 5 to compound 7 is computed to be endothermic (3.3 kcal/mol). We were unable to observe the reverse reaction (7 → 5) under the photochemical conditions

Rotational Spectroscopy of 1‑Cyano-2-methylenecyclopropane (C₅H₅N)A Newly Synthesized Pyridine Isomer

The gas-phase rotational spectrum of 1-cyano-2-methylenecyclopropane (C1, C5H5N), an isomer of pyridine, is presented for the first time, covering the range from 235 to 500 GHz. Over 3600 a-, b-, and c-type transitions for the ground vibrational state have been assigned, measured, and least-squares fit to partial-octic A- and S-reduced distorted-rotor Hamiltonians with low statistical uncertainty (σfit = 42 kHz). Transitions for the two lowest-energy fundamental states (ν27 and ν26) and the lowest-energy overtone (2ν27) have been similarly measured, assigned, and least-squares fit to single-state Hamiltonians. Computed vibration–rotation interaction constants (B0–Bv) using the B3LYP and MP2 levels of theory are compared with the corresponding experimental values. Based upon our preliminary analysis, the next few vibrationally excited states form one or more complex polyads of interacting states via Coriolis and anharmonic coupling. The spectroscopic constants and transition frequencies presented here form the foundation for both future laboratory spectroscopy and astronomical searches for 1-cyano-2-methylenecyclopropane

Evidence of a Nitrene Tunneling Reaction: Spontaneous Rearrangement of 2‑Formyl Phenylnitrene to an Imino Ketene in Low-Temperature Matrixes

Triplet 2-formyl phenylnitrene was gen­erated by photolysis of 2-formyl phenylazide isolated in Ar, Kr, and Xe matrixes and characterized by IR, UV–vis, and EPR spectroscopies. Upon generation at 10 K, the triplet nitrene spontaneously rearranges in the dark to singlet 6-imino-2,4-cyclohexadien-1-ketene on the time scale of several hours. The intramolecular [1,4] H atom shift from the nitrene to the imino ketene occurs by tunneling, on the triplet manifold, followed by intersystem crossing. This case constitutes the first direct evidence of a tunneling reaction involving a nitrene

Photochemistry of Furyl- and Thienyldiazomethanes: Spectroscopic Characterization of Triplet 3-Thienylcarbene

Photolysis (λ > 543 nm) of 3-thienyldiazomethane (<b>1</b>), matrix isolated in Ar or N₂ at 10 K, yields triplet 3-thienylcarbene (<b>13</b>) and α-thial-methylenecyclopropene (<b>9</b>). Carbene <b>13</b> was characterized by IR, UV/vis, and EPR spectroscopy. The conformational isomers of 3-thienylcarbene (<i>s</i>-<i>E</i> and <i>s</i>-<i>Z</i>) exhibit an unusually large difference in zero-field splitting parameters in the triplet EPR spectrum (|<i>D</i>/<i>hc</i>| = 0.508 cm^–1, |<i>E</i>/<i>hc</i>| = 0.0554 cm^–1; |<i>D</i>/<i>hc</i>| = 0.579 cm^–1, |<i>E</i>/<i>hc</i>| = 0.0315 cm^–1). Natural Bond Orbital (NBO) calculations reveal substantially differing spin densities in the 3-thienyl ring at the positions adjacent to the carbene center, which is one factor contributing to the large difference in <i>D</i> values. NBO calculations also reveal a stabilizing interaction between the sp orbital of the carbene carbon in the <i>s</i>-<i>Z</i> rotamer of <b>13</b> and the antibonding σ orbital between sulfur and the neighboring carbonan interaction that is not observed in the <i>s</i>-<i>E</i> rotamer of <b>13</b>. In contrast to the EPR spectra, the electronic absorption spectra of the rotamers of triplet 3-thienylcarbene (<b>13</b>) are indistinguishable under our experimental conditions. The carbene exhibits a weak electronic absorption in the visible spectrum (λ_max = 467 nm) that is characteristic of triplet arylcarbenes. Although studies of 2-thienyldiazomethane (<b>2</b>), 3-furyldiazomethane (<b>3</b>), or 2-furyldiazomethane (<b>4</b>) provided further insight into the photochemical interconversions among C₅H₄S or C₅H₄O isomers, these studies did not lead to the spectroscopic detection of the corresponding triplet carbenes (2-thienylcarbene (<b>11</b>), 3-furylcarbene (<b>23</b>), or 2-furylcarbene (<b>22</b>), respectively)