10 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 singletâ€“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 (Î”<i>H</i><sup>0</sup><sub>rxn</sub> = âˆ’17.3 kcal/mol) to be more stable than tetraethynylcyclobutadiene (Î”<i>H</i><sup>0</sup><sub>rxn</sub> = âˆ’11.7 kcal/mol), which, in turn, is substantially more stable than tetracyanocyclobutadiene (Î”<i>H</i><sup>0</sup><sub>rxn</sub> = +12.7 kcal/mol)

### Rotational Spectroscopy of 1â€‘Cyano-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 (Ïƒfit = 42 kHz).
Transitions for the two lowest-energy fundamental states (Î½27 and Î½26) and the lowest-energy overtone
(2Î½27) have been similarly measured, assigned, and
least-squares fit to single-state Hamiltonians. Computed vibrationâ€“rotation
interaction constants (B0â€“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

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

Broad-band
ultraviolet photolysis (Î» > 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 â†’ 5) under the photochemical conditions

### Rotational Spectroscopy of 1â€‘Cyano-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 (Ïƒfit = 42 kHz).
Transitions for the two lowest-energy fundamental states (Î½27 and Î½26) and the lowest-energy overtone
(2Î½27) have been similarly measured, assigned, and
least-squares fit to single-state Hamiltonians. Computed vibrationâ€“rotation
interaction constants (B0â€“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

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

Broad-band
ultraviolet photolysis (Î» > 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 â†’ 5) under the photochemical conditions

### Ligand-Free Suzukiâ€“Miyaura Coupling Reactions Using an Inexpensive Aqueous Palladium Source: A Synthetic and Computational Exercise for the Undergraduate Organic Chemistry Laboratory

An inexpensive procedure for introducing
the Suzukiâ€“Miyaura coupling reaction into a high-enrollment
undergraduate organic chemistry laboratory course is described. The
procedure employs an aqueous palladium solution as the catalyst and
a range of para-substituted aryl bromides and arylboronic acids as
substrates. The coupling reactions proceed rapidly at room temperature
using standard glassware and do not require ligands, an inert atmosphere,
or specialized equipment. Computational chemistry is used to explore
the molecular and electronic structures of typical starting materials
and products of the Suzukiâ€“Miyaura coupling

### Millimeter-Wave Spectroscopy, Xâ€‘ray Crystal Structure, and Quantum Chemical Studies of Diketene: Resolving Ambiguities Concerning the Structure of the Ketene Dimer

The pure rotational spectrum of diketene
has been studied in the
millimeter-wave region from âˆ¼240 to 360 GHz. For the ground
vibrational state and five vibrationally excited satellites (Î½<sub>24</sub>, 2Î½<sub>24</sub>, 3Î½<sub>24</sub>, 4Î½<sub>24</sub>, and Î½<sub>16</sub>), the observed spectrum allowed
for the measurement, assignment, and least-squares fitting a total
of more than 10â€¯000 distinct rotational transitions. In each
case, the transitions were fit to single-state, complete or near-complete
sextic centrifugally distorted rotor models to near experimental error
limits using Kisielâ€™s ASFIT. Additionally, we obtained less
satisfactory least-squares fits to single-state centrifugally distorted
rotor models for three additional vibrational states: Î½<sub>24</sub> + Î½<sub>16</sub>, Î½<sub>23</sub>, and 5Î½<sub>24</sub>. The structure of diketene was optimized at the CCSDÂ(T)/ANO1
level, and the vibrationâ€“rotation interaction (Î±<sub><i>i</i></sub>) values for each normal mode were determined
with a CCSDÂ(T)/ANO1 VPT2 anharmonic frequency calculation. These Î±<sub><i>i</i></sub> values were helpful in identifying the previously
unreported Î½<sub>16</sub> and Î½<sub>23</sub> fundamental
states. We obtained a single-crystal X-ray structure of diketene at
âˆ’173 Â°C. The bond distances are increased in precision
by more than an order of magnitude compared to those in the 1958 X-ray
crystal structure. The improved accuracy of the crystal structure
geometry resolves the discrepancy between previous computational and
experimental structures. The rotational transition frequencies provided
herein should be useful for a millimeter-wave or terahertz search
for diketene in the interstellar medium

### Millimeter-Wave Spectroscopy, Xâ€‘ray Crystal Structure, and Quantum Chemical Studies of Diketene: Resolving Ambiguities Concerning the Structure of the Ketene Dimer

The pure rotational spectrum of diketene
has been studied in the
millimeter-wave region from âˆ¼240 to 360 GHz. For the ground
vibrational state and five vibrationally excited satellites (Î½<sub>24</sub>, 2Î½<sub>24</sub>, 3Î½<sub>24</sub>, 4Î½<sub>24</sub>, and Î½<sub>16</sub>), the observed spectrum allowed
for the measurement, assignment, and least-squares fitting a total
of more than 10â€¯000 distinct rotational transitions. In each
case, the transitions were fit to single-state, complete or near-complete
sextic centrifugally distorted rotor models to near experimental error
limits using Kisielâ€™s ASFIT. Additionally, we obtained less
satisfactory least-squares fits to single-state centrifugally distorted
rotor models for three additional vibrational states: Î½<sub>24</sub> + Î½<sub>16</sub>, Î½<sub>23</sub>, and 5Î½<sub>24</sub>. The structure of diketene was optimized at the CCSDÂ(T)/ANO1
level, and the vibrationâ€“rotation interaction (Î±<sub><i>i</i></sub>) values for each normal mode were determined
with a CCSDÂ(T)/ANO1 VPT2 anharmonic frequency calculation. These Î±<sub><i>i</i></sub> values were helpful in identifying the previously
unreported Î½<sub>16</sub> and Î½<sub>23</sub> fundamental
states. We obtained a single-crystal X-ray structure of diketene at
âˆ’173 Â°C. The bond distances are increased in precision
by more than an order of magnitude compared to those in the 1958 X-ray
crystal structure. The improved accuracy of the crystal structure
geometry resolves the discrepancy between previous computational and
experimental structures. The rotational transition frequencies provided
herein should be useful for a millimeter-wave or terahertz search
for diketene in the interstellar medium

### Toward Understanding the Decomposition of Carbonyl Diazide (N<sub>3</sub>)<sub>2</sub>Cî—»O and Formation of Diazirinone <i>cycl</i>-N<sub>2</sub>CO: Experiment and Computations

Carbonyl diazide, (N<sub>3</sub>)<sub>2</sub>CO (I), is a highly explosive compound. The isolation of the
substance in a neat form was found to provide unique access to two
other high-energy molecules, namely, N<sub>3</sub>â€“NCO (III)
and <i>cycl</i>-N<sub>2</sub>CO (IV), among the decomposition
products of (I). To understand the underlying reaction mechanism,
the decomposition reactions including the thermal conversion of two
conformers of (I) were revisited, and the potential energy surface
(PES) was computationally explored by using the methods of B3LYP/6-311+GÂ(3df)
and CBS-QB3. The most stable synâ€“syn structure (I) readily
converts into the synâ€“anti conformer (Î”<i>H</i><sub>exptl</sub> = 1.1 Â± 0.5 kcal mol<sup>â€“1</sup>),
which undergoes decomposition in two competing pathways: a concerted
path to N<sub>3</sub>â€“NCO (III) or a stepwise route to (III)
via the nitrene intermediate N<sub>3</sub>CÂ(O)N <sup>1</sup>(II).
The calculated activation barriers (<i>E</i><sub>a</sub>) are almost the same (âˆ¼33 kcal mol<sup>â€“1</sup>, B3LYP/6-311+GÂ(3df)).
Further decomposition of (III) occurs through a concerted fragmentation
into 2 N<sub>2</sub> + CO with a moderate <i>E</i><sub>a</sub> of 22 kcal mol<sup>â€“1</sup>, and this process is compared
to the isoelectronic species N<sub>3</sub>â€“N<sub>3</sub> â†’
3 N<sub>2</sub> (<i>E</i><sub>a</sub> = 17 kcal mol<sup>â€“1</sup>) and OCNâ€“NCO â†’ N<sub>2</sub> + 2 CO
(61 kcal mol<sup>â€“1</sup>). No low-energy pathway leading to
(IV) was found on the singlet PES. However, the intervention of triplet
ground-state <sup>3</sup>(II) from the initially generated <sup>1</sup>(II) through an intersystem crossing (ISC) offers a likely approach
to (IV); that is, <sup>3</sup>(II) can decompose in a concerted process
(<i>E</i><sub>a</sub> = 30 kcal mol<sup>â€“1</sup>)
by eliminating one N<sub>2</sub> to yield the disfavored OCNN <sup>3</sup>(VI). A careful intrinsic reaction coordinate analysis and
a combined energy scan of the Nâ€“Câ€“N angle reveals a
bifurcation point on this triplet PES, which allows a spin crossover
to the singlet PES along the reaction coordinate and eventually leads
to the formation of the metastable diazirinone (IV)

### Carbonyl Diazide, OC(N<sub>3</sub>)<sub>2</sub>: Synthesis, Purification, and IR Spectrum

Carbonyl diazide (<b>1</b>), OCÂ(N<sub>3</sub>)<sub>2</sub>, is prepared by reaction of triphosgene and tetra-<i>n</i>-butylammonium azide in a solution of diethyl ether or
dimethyl ether.
The advantage of this synthetic method, relative to other procedures,
is that the use of triphosgene, OCÂ(OCCl<sub>3</sub>)<sub>2</sub>,
mitigates the need to use highly poisonous reagents such as phosgene,
OCCl<sub>2</sub>, or chlorofluorocarbonyl, OCÂ(Cl)ÂF. The identity and
purity of OCÂ(N<sub>3</sub>)<sub>2</sub> are established by gas-phase
IR spectroscopy, which reveals the presence of both <i>synâ€“syn</i> and <i>antiâ€“syn</i> conformers. Computed anharmonic
vibrational frequencies and infrared intensities of carbonyl diazide
(<b>1</b>) display excellent agreement with experiment, and
reveal the presence of strong Fermi resonances