26 research outputs found

### 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 singletātriplet 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 (Ī<i>H</i><sup>0</sup><sub>rxn</sub> = ā17.3 kcal/mol) to be more stable than tetraethynylcyclobutadiene (Ī<i>H</i><sup>0</sup><sub>rxn</sub> = ā11.7 kcal/mol), which, in turn, is substantially more stable than tetracyanocyclobutadiene (Ī<i>H</i><sup>0</sup><sub>rxn</sub> = +12.7 kcal/mol)

### Photochemistry of CpMn(CO)<sub>3</sub> and Related Derivatives: Spectroscopic Observation of Singlet and Triplet CpMn(CO)<sub>2</sub>

We report novel photochemistry derived from (Ī·<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)ĀMnĀ(CO)<sub>3</sub> (<b>1a</b>),
(Ī·<sup>5</sup>-C<sub>5</sub>H<sub>4</sub>Me)ĀMnĀ(CO)<sub>3</sub> (<b>1b</b>), (Ī·<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)ĀMnĀ(CO)<sub>3</sub> (<b>1c</b>), and (Ī·<sup>5</sup>-indenyl)ĀMnĀ(CO)<sub>3</sub> (<b>1d</b>). Photolysis (>261 nm, 1 h) of the parent
tricarbonyl (<b>1a</b>ā<b>d</b>), matrix isolated
in argon at 10 K, yields two species: the expected singlet dicarbonyl <sup>1</sup>(Ī·<sup>5</sup>-L)ĀMnĀ(CO)<sub>2</sub> (<sup><b>1</b></sup><b>2a</b>ā<b>d</b>) and an additional compound
assigned as the triplet dicarbonyl <sup>3</sup>(Ī·<sup>5</sup>-L)ĀMnĀ(CO)<sub>2</sub> (<sup><b>3</b></sup><b>2a</b>ā<b>d</b>). Density functional theory calculations (B3LYP/LANL2DZ)
support the structural assignments for <sup><b>1</b></sup><b>2</b> and <sup><b>3</b></sup><b>2</b>. Natural bond
orbital population analyses of <sup><b>1</b></sup><b>2a</b> and <sup><b>3</b></sup><b>2a</b> explain the source
of the large coupling (ĪĪ½<sub>CO</sub> 153 cm<sup>ā1</sup>) between the carbonyl stretching vibrations in <sup><b>3</b></sup><b>2a</b>. The triplet isomer (<sup><b>3</b></sup><b>2</b>) is metastable, even at temperatures as low as 10
K. We determined the rate constants for the thermal isomerization <sup><b>3</b></sup><b>2 </b>ā <sup><b>1</b></sup><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 ā« Cp ā Cpā²
> Cp*. The spectroscopy and kinetics observed in various matrices
(Ar, CH<sub>4</sub>, and Xe) do not differ appreciably. Experimental
and computational results suggest that the singletātriplet
energy gap (Ī<i>E</i><sub>ST</sub>) of CpMnĀ(CO)<sub>2</sub> (<b>2a</b>) must be smaller than previous estimates

### Rotational Spectroscopy of 1āCyano-2-methylenecyclopropane (C<sub>5</sub>H<sub>5</sub>N)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 (Ļfit = 42 kHz).
Transitions for the two lowest-energy fundamental states (Ī½27 and Ī½26) and the lowest-energy overtone
(2Ī½27) have been similarly measured, assigned, and
least-squares fit to single-state Hamiltonians. Computed vibrationārotation
interaction constants (B0āBv) 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 Ć 10<sup>ā5</sup> s<sup>ā1</sup> even in
the dark at 10 K, despite a computed activation barrier of 3.4 kcal
mol<sup>ā1</sup>. Because this rate is 10<sup>57</sup> 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 2āFormylphenylnitrene: 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 2āformylphenylnitrene isolated
in an argon matrix. The ring-expansion reaction takes place at 10
K with a rate constant of ā¼7.4 Ć 10<sup>ā7</sup> s<sup>ā1</sup>, despite an estimated activation barrier of
7.5 kcal mol<sup>ā1</sup>. 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 (C<sub>5</sub>H<sub>5</sub>N) to 1āCyano-2-methylenecyclopropane in an Argon Matrix

Broad-band
ultraviolet photolysis (Ī» > 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 ā 5) under the photochemical conditions

### Photoisomerization of (Cyanomethylene)cyclopropane (C<sub>5</sub>H<sub>5</sub>N) to 1āCyano-2-methylenecyclopropane in an Argon Matrix

Broad-band
ultraviolet photolysis (Ī» > 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 ā 5) under the photochemical conditions

### Rotational Spectroscopy of 1āCyano-2-methylenecyclopropane (C<sub>5</sub>H<sub>5</sub>N)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 (Ļfit = 42 kHz).
Transitions for the two lowest-energy fundamental states (Ī½27 and Ī½26) and the lowest-energy overtone
(2Ī½27) have been similarly measured, assigned, and
least-squares fit to single-state Hamiltonians. Computed vibrationārotation
interaction constants (B0āBv) 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 2āFormyl Phenylnitrene to an Imino Ketene in Low-Temperature Matrixes

Triplet
2-formyl phenylnitrene was genĀerated by photolysis
of 2-formyl phenylazide isolated in Ar, Kr, and Xe matrixes and characterized
by IR, UVāvis, 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 (Ī» > 543 nm) of 3-thienyldiazomethane
(<b>1</b>), matrix isolated in Ar or N<sub>2</sub> at 10 K,
yields triplet
3-thienylcarbene (<b>13</b>) and Ī±-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<sup>ā1</sup>, |<i>E</i>/<i>hc</i>| = 0.0554 cm<sup>ā1</sup>; |<i>D</i>/<i>hc</i>| = 0.579 cm<sup>ā1</sup>, |<i>E</i>/<i>hc</i>| = 0.0315 cm<sup>ā1</sup>). 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
Ļ orbital between sulfur and the neighboring carbonīøan
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
(Ī»<sub>max</sub> = 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 C<sub>5</sub>H<sub>4</sub>S or C<sub>5</sub>H<sub>4</sub>O 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)