Spectroscopy and Photochemistry of Triplet 1,3-Dimethylpropynylidene (MeC3Me)

Article abstract of DOI:10.1021/jacs.6b07444

Photolysis (λ > 472 nm) of 2-diazo-3-pentyne (11) affords triplet 1,3-dimethylpropynylidene (MeC₃Me, ³3), which was characterized spectroscopically in cryogenic matrices. The infrared, electronic absorption, and electron paramagnetic resonance spectra of MeC₃Me (³3) are compared with those of the parent system (HC₃H) to ascertain the effect of alkyl substituents on delocalized carbon chains of this type. Quantum chemical calculations (CCSD(T)/ANO1) predict an unsymmetrical equilibrium structure for triplet MeC₃Me (³3), but they also reveal a very shallow potential energy surface. The experimental IR spectrum of triplet MeC₃Me (³3) is best interpreted in terms of a quasilinear, axially symmetric structure. EPR spectra yield zero-field splitting parameters that are typical for triplet carbenes with axial symmetry (|D/hc| = 0.63 cm^-1, |E/hc| = β 0 cm^-1), while theoretical analysis suggests that the methyl substituents confer significant spin polarization to the carbon chain. Upon irradiation into the near-UV electronic absorption (λ_max 350 nm), MeC₃Me (³3) undergoes 1,2-hydrogen migration to yield pent-1-en-3-yne (4), a photochemical reaction that is typical of carbenes bearing a methyl substituent. This facile process apparently precludes photoisomerization to other interesting C₅H₆ isomers, in contrast to the rich photochemistry of the parent C₃H₂ system.

Full text of DOI:10.1021/jacs.6b07444

Article
pubs.acs.org/JACS
Spectroscopy and Photochemistry of Triplet 1,3-
Dimethylpropynylidene (MeC Me)
3
Stephanie N. Knezz, Terese A. Waltz, Benjamin C. Haenni, Nicola J. Burrmann, and Robert J. McMahon*
Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706-1322, United
States
*
S Supporting Information
ABSTRACT: Photolysis (λ > 472 nm) of 2-diazo-3-pentyne (11)
3
affords triplet 1,3-dimethylpropynylidene (MeC Me, 3), which
3
was characterized spectroscopically in cryogenic matrices. The
infrared, electronic absorption, and electron paramagnetic reso-
3
nance spectra of MeC Me ( 3) are compared with those of the
3
parent system (HC H) to ascertain the effect of alkyl substituents
3
on delocalized carbon chains of this type. Quantum chemical
calculations (CCSD(T)/ANO1) predict an unsymmetrical equili-
3
brium structure for triplet MeC Me ( 3), but they also reveal a very shallow potential energy surface. The experimental IR
3
3
spectrum of triplet MeC Me ( 3) is best interpreted in terms of a quasilinear, axially symmetric structure. EPR spectra yield zero-
3
−
1
−1
field splitting parameters that are typical for triplet carbenes with axial symmetry (|D/hc| = 0.63 cm , |E/hc| = ∼ 0 cm ), while
theoretical analysis suggests that the methyl substituents confer significant spin polarization to the carbon chain. Upon irradiation
into the near-UV electronic absorption (λmax 350 nm), MeC Me ( 3) undergoes 1,2-hydrogen migration to yield pent-1-en-3-yne
3
3
(
4), a photochemical reaction that is typical of carbenes bearing a methyl substituent. This facile process apparently precludes
photoisomerization to other interesting C H isomers, in contrast to the rich photochemistry of the parent C H system.
5
6
3
2
3
INTRODUCTION
isomers (Scheme 2). 1,3-Diphenylpropynylidene (PhC Ph; 2),
3
■
the most carefully studied substituted derivative of propynyli-
The study of acetylenic carbenes offers a deeper understanding
of the electronic structure and chemical reactivity of conjugated
carbon chain molecules. Propynylidene (HCCCH; 1), the
3
Scheme 2
simplest acetylenic carbene, is an intermediate in reactions
1
,2
relevant to astrochemistry
and combustion of fuel-rich
3
,4
hydrocarbon flames. The geometry and electronic structure
of triplet propynylidene were once debated, but it is now
3
generally accepted that 1 exists as a quasilinear species with C
2
5
−8
equilibrium geometry (Scheme 1).
The electronic structure
of 1 is best described as a nearly equal admixture of carbenic
1,1-diradical) and allenic (1,3-diradical) character (Scheme
3
(
1
7
).
The case of triplet propynylidene notwithstanding, the study
of acetylenic carbenes inevitably confronts the possibility of a
localized carbene structure and the existence of bond-shift
dene, exhibits a symmetrical, allenic diradical structure with a
triplet ground state. In the current investigation, we describe
9
experimental and computational studies of 1,3-dimethylpropy-
3
nylidene (MeC Me) ( 3). Although spectroscopic character-
a
3
Scheme 1
3
ization (IR, UV/vis, and EPR) of MeC Me ( 3) under matrix
3
isolation conditions reveals many similarities to the parent
HC H system, theory predicts that symmetrical substitution of
3
HC H with two methyl substituents leads to an unsymmetrical,
3
localized carbenic structure. The studies described herein
answer some questions, and raise others, concerning the
3
structure of triplet MeC Me ( 3).
3
aEquilibrium structure of 31 computed at CCSD(T)/cc-pVTZ;
assessment of resonance contributors by Natural Resonance Theory
at QCISD/6-311+G(2df,p).
7
Received: July 19, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b07444
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Journal of the American Chemical Society
Article
RESULTS AND DISCUSSION
Computational Results. Relative energies of several
■
pertinent C H isomers were computed using coupled-cluster
5
6
and density functional theory methods. Coupled-cluster values
CCSD/cc-pVDZ) are provided in Figure 1. (T diagnostic
(
1
Figure 2. Computed structures for triplet and singlet MeC Me (3).
3
3
3
(
3
a) 3 at B3LYP/6-31G(d) (D ). (b) 3 at CCSD/cc-pVDZ (C ). (c)
3
s
1
3
at CCSD/cc-pVDZ (C ). (d) 3 at CCSD(T)/ANO1 (C ).
1 1
1
3
3
11G(d,p). ) Our DFT studies find that D and D3h
3d
structures each exhibit one imaginary vibrational frequency.
At the CCSD/cc-pVDZ level, the carbon chain is bent and
adopts an unsymmetrical, carbene-like structure in which the
Figure 1. Computed relative energy (kcal/mol; CCSD/cc-pVDZ,
including ZPVE) of selected C H isomers.
5
6
methyl groups are nonequivalent (C geometry) (Figure 2b). At
s
values, which may be used to assess the multi-reference
the highest level of theory employed in this study (CCSD(T)/
ANO1), a further distortion involving the dihedral angle of one
of the methyl groups relative to the CCC plane removes all
10,11
character of the CCSD wave function,
Supporting Information.) DFT relative energies (B3LYP/6-
1G(d)) are also provided in Supporting Information. Our
are provided in
3
symmetry (C geometry) (Figure 2d). In contrast to these
1
3
computational data, involving both optimized structures and
total electronic energies, exhibit poor agreement with the DFT
subtle complications associated with triplet MeC Me ( 3), the
computed structure of singlet MeC Me ( 3) is less sensitive to
3
1
3
1
2
data reported by Kassaee and co-workers. Geometry
optimization of these systems is complicated because of the
presence of two methyl rotors, and we surmise that this factor is
responsible for the differences. In our study, we expended
considerable effort to ensure that computed structures exhibit
real vibrational frequencies (no imaginary frequencies) and thus
represent true minima on the potential energy surface.
the level of theory. At the CCSD/cc-pVDZ level, the structure
1
of singlet MeC Me ( 3) is similar to the C structure obtained
3
1
3
for triplet MeC Me ( 3), except for a more pronounced C−C−
3
3
1
CH angle at the “carbene” carbon (155° in 3 vs 114° in 3)
3
(Figure 2c). The computed singlet−triplet energy splitting for
MeC Me is 11.8 kcal/mol (CCSD/cc-pVDZ), which is quite
3
similar to the experimentally measured value of 11.5 kcal/mol
8
The relative energies of the dimethyl-substituted derivatives
are qualitatively similar to those of the parent C H system.
for the parent HC H. The subtleties in structure for RC R
3
3
systems make nomenclature challenging. The singlet and triplet
3
2
The major difference upon methyl substitution appears to be
that singlets 5 and 6 experience a greater stabilization than
states of MeC Me are each described by important resonance
3
contributions from both carbene (propynylidene nomenclature:
1,3-dimethylpropynylidene or pent-3-yn-2-ylidene) and dirad-
ical (allene-diyl nomenclature: 1,3-dimethylallen-1,3-diyl or
penta-2,3-dien-2,4-diyl) structures. For this reason, we often
triplet 3. The computed energy separation between singlet 2,3-
3
dimethylcyclopropenylidene (5) and triplet MeC Me ( 3)
3
increases to 14.4 kcal/mol (from 9.8 kcal/mol for cyclo-
propenylidene vs triplet HC H) while the separation between
refer to the species by formula (MeC Me) rather than by name.
3
3
3
triplet MeC Me ( 3) and 3,3-dimethylpropadienylidene (6)
Carbene Precursor. The preparation of 2-diazo-3-pentyne
3
decreases to 0.1 kcal/mol (from 3.4 kcal/mol for triplet HC H
(11) was not as straightforward as we might have imagined.
3
6
vs propadienylidene).
Pent-3-yn-2-one (8) was prepared according to literature
3
14,15
The computed structures of triplet MeC Me ( 3) exhibit
procedures (Scheme 3).
Tosylhydrazone formation is
3
subtle differences that depend on the computational method
and basis set employed (Figure 2). Density functional methods
occasionally problematic with simple ynals or ynones because
of either conjugate addition or intramolecular cyclization of the
initially formed tosylhydrazone to yield a pyrazole deriva-
(
B3LYP) predict a symmetrical delocalized structure (D3
16,17
symmetry) in which the deviation from an idealized D3d
structure varies with basis set (Figure 2a). (Huang et al.
reported D3d for the optimized structure at B3LYP/6-
tive.
In the current case, tosylhydrazone formation seemed
to proceed normally, but subsequent treatment of the adduct
with NaH under usual reaction conditions did not afford the
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Scheme 3
Scheme 4
sodium salt. Although we were able to prepare the lithium salt
of the tosylhydrazone with n-BuLi, thermolysis of the salt with
matrix isolation trapping of the products afforded diazo
compound 11 in low yield and impure form, as judged by
the matrix IR spectrum. Thus, we turned to the preparation of
the trisylhydrazone derivativean approach that we employed
9
in the related case of 1-diazo-1,3-diphenyl-2-propyne. Gentle
thermolysis of the sodium salt of trisylhydrazone 10 smoothly
afforded 2-diazo-3-pentyne (11).
Spectroscopy and Photochemistry of MeC Me ( 3).
3
3
Photolysis (λ > 472 nm) of 2-diazo-3-pentyne (11) under
matrix isolation conditions (Ar or N , 10 K) produces
2
3
spectroscopic features attributable to triplet MeC Me ( 3)
3
(
Scheme 4), as observed by IR, UV/vis, and EPR spectroscopy.
3
Triplet MeC Me ( 3) is photosensitive to near-UV irradiation
3
(
λ > 330 nm), undergoing an intramolecular 1,2-hydrogen shift
3
to produce pent-1-en-3-yne (4). In the following sections, we
Figure 3. Top: Computed IR spectra for triplet MeC
3
Me ( 3) at
CCSD(T)/ANO1 (above) and B3LYP/6-31G(d) (below). Middle: IR
subtraction spectrum showing the disappearance of 2-diazo-3-pentyne
11) and appearance of triplet MeC Me ( 3) upon irradiation (λ >
72 nm, 6 h, N , 10 K). Bottom: Computed spectrum for 2-diazo-3-
will discuss the details of the spectroscopic characterization of
3
3
.
3
(
3
IR Spectroscopy. Photolysis (λ > 472 nm, 6 h; N , 10 K) of
2
4
2
2
-diazo-3-pentyne (11) results in the disappearance of IR
pentyne (11) (CCSD/cc-pVDZ).
absorptions associated with the diazo compound and the
appearance of new bands that can be attributed to triplet
3
MeC Me ( 3) (Figure 3). The new IR bands are inconsistent
3
with those predicted for the diazirine isomer of 2-diazo-3-
spectrum occurs in a region of the spectrum that is often
pentyne (11) (Supporting Information). It should be noted
contaminated by water. We are not confident in assigning this
3
3
that the computed IR spectra of triplet MeC Me ( 3) presented
feature to triplet MeC Me ( 3) and note that the agreement
3
3
−1
in Figure 3 reflect not merely different levels of computational
theory, they represent different predicted structures. The high
with the putative feature at 1818 cm in the predicted
spectrum is much poorer than would be expected for a
computation at this high level of theory. Thus, our tentative
symmetry structure (D ) predicted by B3LYP/6-31G(d) does
3
not exhibit an IR-active CCC symmetric stretching
conclusion is that the absence of a CCC symmetric
3
vibration, while the low symmetry structure (C ) predicted
vibration in the IR spectrum of triplet MeC Me ( 3) implies
1
3
−1
by CCSD(T)/ANO1 does (1818 cm ). Neither computa-
either a high-symmetry structure or a quasilinear molecular
structure, in which degenerate unsymmetrical structures
equilibrate because the barrier to linearity lies below the zero-
tional method takes into account the effects of anharmonicity.
−1
The unusually sharp peak at 1598 cm in the experimental
C
DOI: 10.1021/jacs.6b07444
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Journal of the American Chemical Society
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point energy. The parent HC H is experimentally known to be
3
7
,8
a quasilinear molecule.
Subsequent irradiation (λ > 330 nm, 24 h, N , 10 K) of the
2
3
matrix containing triplet MeC Me ( 3) causes the disappear-
3
ance of the IR absorptions associated with the triplet species
and the appearance of new absorptions (Figure 4). The identity
Figure 4. Top: Authentic IR spectrum of matrix isolated pent-1-en-3-
yne (4) (N , 10 K). Middle: IR subtraction spectrum showing the
2
3
disappearance of triplet MeC Me ( 3) and appearance of pent-1-en-3-
3
yne (4) upon irradiation (λ > 330 nm, 24 h, N , 10 K) (D = residual
2
diazo compound 11). Bottom: Computed IR spectrum of triplet
3
MeC Me ( 3) (B3LYP/6-31G(d)).
3
of the photoproduct is assigned as pent-1-en-3-yne (4), the
product of 1,2-hydrogen migration in MeC Me, by comparison
3
with an IR spectrum of an authentic sample of enyne 4. To
determine whether the 1,2-hydrogen shift reaction might also
proceed via a tunneling mechanism, we allowed a matrix
3
containing triplet MeC Me ( 3) to stand in the dark at 10 K.
3
After 24 h, no changes were observed in the IR spectrum,
thereby establishing that a hydrogen atom tunneling process is
not operative.
Figure 5. Top: Electronic absorption spectrum of 2-diazo-3-pentyne
(
(
11) after deposition (60 min, N , 13 K). Middle: Triplet MeC Me
2
3
3
3) obtained upon irradiation of 11 (λ > 472 nm, 11 h). Bottom:
UV/vis Spectroscopy. Photolysis (λ > 472 nm, 11 h, N2,
3 K) of 2-diazo-3-pentyne (11) results in bleaching of the
3
Pent-1-en-3-yne (4) obtained upon irradiation of 3 (λ > 330 nm, 3
1
h).
strong UV absorption associated with the diazo compound
(
(
(
λmax 250 nm) and appearance of a weak near-UV absorption
3
λmax 350 nm) that can be attributed to triplet MeC Me ( 3)
3
Figure 5). The position and relative intensity of the electronic
3
absorption of triplet MeC Me ( 3) is quite similar to that
observed previously for triplet HC H ( 1), with the absorption
of MeC Me ( 3) exhibiting a slight redshift (λ 350 nm vs
3
3
3
3
3
3
max
6
10 nm). In analogy to the behavior observed in the IR
experiments, photoexcitation (λ > 330 nm, 3 h) into the near-
UV electronic absorption (λ 350 nm) of triplet MeC Me
max
3
3
3
(
3) results in efficient bleaching of the absorption of 3 and
the appearance of a strong UV absorption attributable to pent-
1
-en-3-yne (4), the product of 1,2-hydrogen migration (Figure
3
5). When a matrix containing triplet MeC Me ( 3) was allowed
3
to stand in the dark for 24 h, no changes occurred in the
electronic absorption spectrum.
EPR Spectroscopy. Photolysis (λ > 613 nm, 13.75 h; Ar,
1
0 K) of 2-diazo-3-pentyne (11) affords the triplet EPR
3
3
spectrum of MeC Me ( 3) (Figure 6). The appearance of two
Figure 6. EPR spectrum of triplet MeC Me ( 3) after irradiation of 2-
3
3
sets of signals in the EPR spectrum (major and minor) could be
a result of a matrix site effect or the presence of distinct
conformational isomers. An axially symmetric structure for
diazo-3-pentyne (11) (λ > 613 nm, 13.75 h; Ar, 10 K).
D
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Journal of the American Chemical Society
Article
3
a
MeC Me ( 3) should exhibit a single XY transition rather than
Table 1. Computed Energies for Triplet MeC Me (3)
3
3
2
distinct X and Y transitions. That being said, the deviation
from axial symmetry is not large. The issues of matrix effects
and relaxed vs nonrelaxed structures have been dealt with, in
considerable detail, for the closely related cases of HC H ( 1)
2
2
E
(rel)
E + ZPVE
(rel)
b
c
d
c
d
symmetry N(i)
E
E + ZPVE
3
7
C1
0
5
5
5
−193.626328
−193.626001
−193.626000
−193.626003
0.00
0.21
0.21
0.20
−193.541322
−193.541975
−193.541975
−193.541573
0.00
−0.41
−0.41
−0.16
3
9
D3
3
and PhC Ph ( 2).
3
D3h
The values of the zero-field splitting parameter D (major: |D/
D3d
hc| = 0.627 cm− ; minor: 0.651 cm ) are comparable to the
1;
−1
a
−1
6,7
CCSD(T)/ANO1; D , D , D geometry optimizations constrained
3 3h 3d
b
parent HC H (|D/hc| = 0.64 cm ), indicating that alkyl
3
to the given symmetry. Number of imaginary vibrational frequencies.
substituents do not greatly affect the spin density of the π
c
d
Total energy in Hartree. Relative energy in kcal/mol.
system. A similar relationship was observed among the triplets
HC H, MeC H, and MeC Me. While the large D value might
seem to favor a carbenic structure rather than a 1,3-allenic
diradical structure, it is now understood that one-center dipolar
couplings of the unpaired spins at the C-1 and C-3 positions of
the carbon chain give rise to large D values in systems of this
18
5
5
5
3
MeC Me ( 3) resides on an exceptionally shallow portion of
3
the potential energy surface. They also underscore the
challenge of understanding these subtle features in detail. It is
likely that this system is not well treated within the harmonic
approximation and it is possible that it is not well treated with
7
type. The very small values of the zero-field splitting parameter
23
the Born−Oppenheimer approximation.
−1
−1
E (major: |E/hc| = 0.0073 cm ; minor: 0.0048 cm ) are
consistent with small deviations from axial symmetrywhether
they be intrinsic to the structure or imposed by subtle
perturbations of the host matrix.
Subsequent irradiation (λ > 299 nm) of the matrix
containing triplet MeC Me ( 3) causes a substantial decrease
On the basis of the flat potential energy surface and the very
low frequency vibrational mode (8 cm , B3LYP/6-31G(d); 9
cm , CCSD(T)/ANO1), as well as by analogy with related
we infer that triplet MeC Me ( 3) is a quasilinear
This inference lends support to the interpreta-
tion of the IR spectrum, as described above. Even though a
sophisticated electronic structure calculation predicts an
unsymmetrical equilibrium structure for triplet MeC Me ( 3),
the flatness of the potential energy surface affords a quasilinear
structure. Thus, the experimental IR spectrum of triplet
MeC Me ( 3) is best described by in terms of a higher
−1
−1
7,8,24
3
systems,
3
25−29
molecule.
3
3
of the triplet signal, consistent with the photoisomerization of
3
MeC Me ( 3) to pent-1-en-3-yne (4). No new triplet EPR
3
3
3
transitions were observed in subsequent irradiations at shorter
wavelengths.
Structure. Prior experimental
establish symmetrical structures for HC H, HC H, and
HC H. That symmetrical disubstitution of the symmetrical
HC H (1) would result in an unsymmetrical structure for
MeC Me (3) came as an unexpected result to us. For the first
time, we seem to have stumbled into an acetylenic carbene that
favors an unsymmetrical structure. If the molecule is indeed
unsymmetrical, then there must be two equivalent unsym-
metrical structures (bent at one end or the other) (Figure 7).
6
,7,19,20
and theoretical stud-
3
2
1,22
3
ies
3
5
symmetry structure (D ) and not the predicted, low-symmetry
3
7
equilibrium structure (C1).
3
Puzzled by the result that symmetrical disubstitution of the
C -symmetric HC H (1) with methyl substituents gives rise to
an unsymmetrical structure for MeC Me (3), we attempted to
dissect the problem by considering the influence of substituting
HC H (1) with a single methyl group (Scheme 5). Triplet
3
2
3
3
3
Scheme 5
Figure 7. Qualitative depiction of putative automerization process in
3
triplet MeC Me ( 3). CCSD/cc-pVDZ structure is C symmetry.
3
s
3
MeC H ( 12) has been the subject of occasional interest
3
The atomic motion required to transform one structure into
the other is very small. Thus, the putative barrier cannot be very
large. We searched the potential energy surface diligently, but
without success, to find a transition state between these two
equivalent structures. As a measure of assessing the flatness of
the potential energy surface, we performed geometry
optimizations at the CCSD(T)/ANO1 level under the
constraint of D , D , or D symmetry. In each case, the
through the years. The triplet EPR spectrum ascribed to
3
MeC H ( 12a) was reported by Bernheim, Skell, and co-
3
30,31
3
workers in the 1960s.
been characterized as quasilinear by theoretical methods.
Computed structures (DFT) have been reported for 12a,
but attempts to obtain a geometry-optimized structure for the
bond-shift isomer, 12b, converged to 12a. In our current
studies, we replicated these computational results at both DFT
and coupled-cluster levels of theory. With respect to the effect
More recently, MeC H ( 12a) has
3
32
3
32,33
3
3
33
3
3h
3d
structures exhibit five imaginary vibrational frequencies (two
doubly degenerate bends and one stretch), establishing that the
computed energies of these structures represent upper limits
for actual low-energy transition states on the potential energy
surface. The D , D , and D structures are exceedingly close in
of methyl substitution in triplet HC H, the first methyl
3
substituent appears to exert the dominant effect in biasing the
system in favor of the alkynyl-substituted propargylic structure
3
MeCCCH ( 12a). A structure in which the first methyl
3
3h
3d
3
energy to the fully optimized C structure (unsymmetrical
substituent is at the carbenic position MeCCCH ( 12b) is
1
−1
3
3
carbene), lying only 0.2 kcal/mol (70 cm ) higher than the C
not only higher in energy than 12a, 12b is not even a
1
3
structure (Table 1). Upon inclusion of zero-point vibrational
energy, the relative energies actually reverse; the high symmetry
structures lie 0.4 kcal/mol lower in energy than the C1
structure. These calculations demonstrate that triplet
minimum on the potential energy surface. In MeC Me ( 3), the
3
second methyl substituent is forced to occupy the less
energetically favorable carbenic position. We scoured the
electronic structure calculations and natural bond orbital
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Journal of the American Chemical Society
NBO) analyses of these systems for insights into the nature of
these subtle interactions, but we were unable to identify
dominant interactions on this very shallow potential energy
surface.
Article
(
Table 2. Computed Spin Densities for Triplets HC H (1),
3
a
MeC H (12), and MeC Me (3)
3
3
As noted in the Introduction, one of the fascinating
3
characteristics of the electronic structure of triplet HC H ( 1)
3
is the nearly equal admixture of carbenic (1,1-diradical) and
allenic (1,3-diradical) character. Figure 8 presents a comparison
natural spin density
atom
HC H (1)
MeC H (12)
MeC Me (3)
3
3
3
H-1
0.04
0.04
0.04
0.05
H-2
H-3
C-α
H
0.04
0.04
−0.14
−0.14
−0.04
1.24
C-1
C-2
C-3
H
1.36
−0.98
1.70
1.38
−0.93
1.57
−0.40
1.24
−0.04
−0.07
C-β
H-4
H-5
H-6
−0.14
0.03
0.05
0.05
4.42
2.00
Σ (|spin density|)
Σ (spin density)
2.96
2.00
0.96
4.37
1.99
2.38
“
excess” spin density
2.42
a
3
From the natural bond orbital (NBO) analyses of triplets HC H ( 1)
3
7
3
QCISD/6-311+G(2df,p), MeC H ( 12) CCSD(T)/cc-pVTZ//
CCSD/cc-pVDZ, and MeC Me ( 3) CCSD(T)/cc-pVTZ//CCSD-
3
3
3
(T)/ANO1).
appear to be dominated by the first methyl group, which
enforces the alkynyl-substituted propargylic structure, as noted
earlier. Systems displaying significant spin polarization may be
considered to have spin density in “excess” of the spin density
35
formally associated with the number of unpaired electrons.
For the triplet species under consideration, there are two
unpaired electrons and the arithmetic sum of the spin density,
Σ (spin density) affords the expected value of ρ = 2.0. Because
of spin polarization, the sum of the absolute values of the spin
Figure 8. Natural resonance theory (NRT) descriptions of triplets
3
3
3
3
HC H ( 1), MeC H ( 12), and MeC Me ( 3). ( 1, QCISD/6-
3
3
3
3
3
3
11+G(2df,p); 12, CCSD(T)/cc-pVTZ//CCSD/cc-pVDZ; 3,
CCSD(T)/cc-pVTZ//CCSD(T)/ANO1).
densities at each atom, Σ (|spin density|), exceeds the formal
3
spin density by a considerable magnitude. For HC H ( 1), the
3
3
of the natural resonance theory/natural bond orbital analyses
“excess” spin density is ρ = 0.96, while for MeC H ( 12), it is ρ
3
3
3
3
3
for the triplets HC H ( 1), MeC H ( 12), and MeC Me ( 3).
= 2.38, and for MeC Me ( 3) it is ρ = 2.42 (Table 2). Thus, the
3
3
3
3
Although the influence of one or two methyl substituents is not
calculations predict that the introduction of a single methyl
3
dramatic, the effects are a modest enhancement of carbenic
group in triplet HC H ( 1) enhances the spin polarization by
3
3
character of HC H ( 1).
an amount greater than one full electron. The concept of spin
polarization is not merely a theoretical construct; the
consequences are subject to experimental verification, as has
3
NBO analyses also provide insight concerning spin densities
and electron delocalization along the carbon chain in these
3
3
7
systems. Our earlier analysis of triplet HC H ( 1) revealed the
been done in the case of triplet HC H ( 1).
3
3
existence of significant spin polarization along the carbon
backbone, which turned out to be a crucial insight in
developing a suitable rationalization for the magnitude of the
Photochemistry. The family of structures on the C H
3 2
potential energy surface exhibits a rich network of photo-
6
isomerization reactions, many of which involve hydrogen
7
zero-field splitting parameter, D. In the current study, the
migration. In the phenyl-substituted series (C Ph ), the
3
2
degree to which methyl substitution enhances the spin
analogous migration reactions of phenyl substituents are not
3
polarization, relative to HC H ( 1), came as a surprise because
observed, but the photocyclization reaction of triplet PhC Ph to
3
3
9
the effect of methyl substitution was not pronounced in our
form singlet 2,3-diphenylcyclopropenylidene still occurs. In
1
9,34
earlier study of triplets HC H vs MeC H.
Natural spin
analyzing our IR data involving the photochemistry of MeC Me
5
5
3
3
3
3
densities (ρ) for triplets HC H ( 1), MeC H ( 12), and
MeC Me ( 3) are presented in Table 2. Of particular note is
( 3), we checked for the formation of 2,3-dimethylcycloprope-
3
3
3
nylidene (5) but found none. Photoexcitation (λ > 330 nm, 3
3
the substantial increase in negative spin density at the center
carbon of the chain upon substitution by one or two methyl
groups (from ρ = −0.40 in HC H to ρ = −0.98 in MeC H).
h) into the near-UV electronic absorption (λ 350 nm) of
max
3
triplet MeC Me ( 3) affords 1,2-hydrogen migration, to form
3
pent-1-en-3-yne (4), as the dominant photochemical process.
This mode of photochemical reactivity has also been observed
3
3
The effects of two methyl substituents are not additive and
F
DOI: 10.1021/jacs.6b07444
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
3
4
18
20
with MeC H, MeC Me, and MeC H. We explored broad
dried with anhydrous magnesium sulfate, filtered, concentrated under
reduced pressure, and purified using flash column chromatography
5
5
7
range of irradiation wavelengths in the study of triplet MeC Me
3), but no additional photoproducts were observed.
3
3
(CHCl ). Concentration of the appropriate fractions revealed pent-3-
(
3
yn-2-one tosylhydrazone (9) (1.24 g, 4.93 mmol, 53%) as a white
1
powder. H NMR (300 MHz, CDCl ) δ 8.61 (s, 1H), 7.84 (d, J = 8.0
3
CONCLUSIONS
■
Hz, 2H), 7.30 (d, J = 8.0 Hz, 2H), 2.40 (s, 3H), 2.05 (s, 3H), 1.99 (s,
3
13
3
7
H). C NMR (CDCl ) δ 144.0, 136.0, 135.7, 129.6, 127.7, 101.4,
Triplet carbene MeC Me ( 3) has been generated under matrix
isolation conditions by photolysis of 2-diazo-3-pentyne (11) in
N and characterized by IR, UV/vis, and EPR spectroscopy.
Although quantum chemical calculations predict an unsym-
metrical equilibrium structure, they also reveal a shallow
3
3
+
1.2, 22.6, 21.3, 4.4. HRMS (ESI) m/z [M + H] calcd for
C H N O S 251.0854, found 251.0856.
12
15
2
2
2
2
,4,6-Triisopropylbenzenesulfonylhydrazide (“Trisylhydra-
40
zide”). 2,4,6-Triisopropylbenzenesulfonyl chloride (2.05 g, 6.8
mmol) was dissolved in 10 mL THF in a 50 mL, oven-dried round-
bottom flask. The solution was cooled to −5 °C, and hydrazine
monohydrate (1.3 mL, 26.7 mmol) was added dropwise over 15 min.
The solution was stirred at 0 °C for 4 h. Water was added to the
solution until the precipitate dissolved, ether was added to form a
substantial organic layer, and the aqueous layer was removed. The
potential energy surface. The experimental IR spectrum of
3
triplet MeC Me ( 3) is best interpreted in terms of a
3
quasilinear, axially symmetric structure. Theoretical analysis of
the electronic structure of this intriguing molecule suggests that
the methyl substituents confer significant spin polarization to
this small, open-shell, carbon-chain molecule.
organic layer was washed 3 times with brine, dried over MgSO4,
filtered over a pad of Celite, and concentrated under reduced pressure
1
to afford trisylhydrazide (1.74 g, 5.9 mmol, 86%) as a white solid. H
EXPERIMENTAL SECTION
General. Chemicals and solvents were purchased and used without
■
NMR (300 MHz, CDCl ): δ 7.20 (s, 2H), 5.44 (s, br, 1H), 4.15 (sept,
3
J = 6.6 Hz, 2H), 2.92 (sept, J = 6.8 Hz), 1.61 (s, br, 2H), 1.28 (d, J =
6.6 Hz, 12H), 1.26 (d, J = 6.8 Hz, 6H).
1
purification, unless otherwise noted. H NMR spectra (300 or 400
MHz) and 13C NMR spectra (75.4 MHz) were obtained in CDCl3;
chemical shifts (δ) are reported as ppm downfield from internal SiMe4.
Mass spectra were acquired using a high-performance liquid
chromatography electrospray ionization quadrupole mass spectrom-
eter.
41
Pent-3-yn-2-one Trisylhydrazone (10). Trisyl hydrazide (1.56
g, 3.8 mmol) was added to a 100 mL flask containing pent-3-yn-2-one
(8) (0.32 g, 3.8 mmol), and the flask was purged with nitrogen for 15
min. To this flask was added 23 mL glacial acetic acid, and the reaction
was stirred under nitrogen at room temperature for 2 h. The reaction
was quenched by pouring the mixture into a large Erlenmeyer flask
Computational Methods. Equilibrium geometries, harmonic
vibrational frequencies, and infrared intensities were computed using
density functional or coupled-cluster methods. Density functional
calculations were performed using the B3LYP functional and the 6-
1G(d) basis set, as implemented in the Gaussian 09 program suite.
Coupled-cluster calculations were performed at CCSD or CCSD(T)
levels with the cc-pVDZ or ANO1 basis sets, as implemented in the
CFOUR program suite. NBO and NRT calculations were
performed as implemented in the Gaussian program suite.
Pent-3-yn-2-ol (7). 1-Propynyl magnesium bromide (100 mL,
containing a solution of saturated NaHCO slowly and stirring for 30
3
min. The solution was extracted three times with dichloromethane.
The combined organic layers were washed with saturated NaHCO3
3
6
3
solution, dried over MgSO , and the pH checked to ensure a neutral
4
solution. The solution was concentrated to reveal a pale yellow solid.
The solid was purified by column chromatography (SiO , 1:5 EtOAc:
3
7
38
2
hexanes) affording pent-3-yn-2-one trisylhydrazone (10) (0.80 g, 2.2
1
mmol, 58%) as a white solid. H NMR (300 MHz, CDCl ): δ 8.28 (s,
1
4
3
1
1
6
3
H), 7.16 (s, 2H), 4.21 (sept, J = 6.8 Hz, 2H), 2.90 (sept, J = 6.9 Hz,
5
0 mmol) was concentrated at reduced pressure to approximately 50
H), 2.10 (s, 3H), 1.99 (s, 3H), 1.27 (d, J = 6.8 Hz, 12H), 1.26 (d, J =
mL, purged with nitrogen, and cooled to 0 °C. A solution of freshly
distilled and dried acetaldehyde (6 mL, 105 mmol) in 40 mL
anhydrous diethyl ether was added dropwise via syringe under
nitrogen at 0 °C. The flask was warmed to room temperature and
allowed to stir under nitrogen. After 5 h, the reaction was quenched by
addition of 60 mL saturated ammonium chloride solution. The white
precipitate was removed by vacuum filtration. The aqueous layer of the
resulting biphasic solution was extracted twice with diethyl ether. The
combined organic layers were washed with water, followed by brine.
The ether solution was dried over MgSO , filtered, and concentrated
at reduced pressure to reveal a yellow oil. The crude product was
distilled at reduced pressure to afford pent-3-yn-2-ol (7) (2.58 g, 30.7
mmol, 61%). H NMR (300 MHz, CDCl ): δ 4.50 (qq, J = 6.6, 2.1 Hz,
+
.8 Hz, 6H). HRMS (ESI-Q-IT) m/z [M+H] calcd for C H N O S
2
0
31
2
2
63.2101, found 363.2097.
Pent-3-yn-2-one Trisylhydrazone Sodium Salt. To an oven-
dried 25 mL flask, pent-3-yn-2-one trisylhydrazone (10) (0.20 g, 0.57
mmol) and NaH (60% in mineral oil) (0.025 g, 0.62 mmol) were
added and purged with N for 15 min. To this was added 9.0 mL
2
distilled diethyl ether, and the suspension was stirred under N at
2
room temperature. After 90 min, the suspension was filtered, and the
precipitate was thoroughly rinsed with hexane. The white solid was
dried overnight in vacuo.
4
2-Diazo-3-pentyne (11). Pent-3-yn-2-one trisylhydrazone sodium
1
salt (0.15 g, 0.40 mmol) was added to the bottom of an oven-dried
sublimator and placed under vacuum. The coldfinger of the sublimator
was filled with dry ice/acetone and the bottom of the sublimator was
heated to 120 °C. Small pink droplets of 2-diazo-3-pentyne (11) began
to form on the coldfinger within 5 min. The reaction was allowed to
continue for 45 min at which time the coldfinger was removed, and the
diazo compound was rinsed into a deposition tube using freshly
3
1
3
H), 2.46 (s, br, 1H), 1.84 (d, J = 2.1 Hz, 3H), 1.42 (d, J = 6.6 Hz,
H).
3
9
Pent-3-yn-2-one (8). To a flame-dried 100 mL round-bottom
flask were added 20 mL dry CH Cl , 6.55 g BaMnO (25.6 mmol),
and 1.20 mL 3-pentyn-2-ol (7) (12.8 mmol). The mixture was allowed
to stir under nitrogen at room temperature overnight. The mixture was
filtered through a medium fritted funnel to remove BaMnO , and the
solution was concentrated under reduced pressure to reveal pent-3-yn-
2
2
4
distilled diethyl ether that had been dried over CaH . Ether was
2
4
removed in vacuo with the deposition tube cooled to −78 °C to yield
2
-diazo-3-pentyne (11) a bright pink residue at the bottom of the tube.
2
-one (8) (0.76 g, 9.27 mmol, 72% yield) as a gold oil. No purification
1
IR (N , 10 K) 2965 w, 2928 w, 2901 w, 2860 w, 2199 w, 2067 vs, 2004
was necessary. H NMR (300 MHz, CDCl ) δ 2.31 (s, 3H), 2.02 (s,
2
3
H). 13C NMR (CDCl ) 184.9, 90.1, 80.6, 32.9, 4.1.
−1
3
w, 1466 w, 1439 m, 1386 m, 1039 w, 664 w cm (Figure 3); UV/vis
3
Pent-3-yn-2-one Tosylhydrazone (9). To a flask containing 0.76
(N
512, 250 nm (Figure S6).
Triplet 1,3-Dimethylpropynylidene ( 3). IR (N
2897 s, 2830 m, 2725 m, 1443 w, 1429 m, 1369 w, 1311 w, 1002 w
2
, 10 K) λmax 250 nm (Figure S7); UV/vis (MeOH, 298 K) λmax
g (9.23 mmol) pent-2-yn-2-one (8) was added 7.0 mL glacial acetic
acid and 2.80 g (15.1 mmol) tosylhydrazide. The mixture was allowed
to stir under nitrogen at room temperature overnight. The mixture was
poured into a beaker containing 100 mL of saturated sodium
bicarbonate solution, stirred for 15 min, and poured into a separatory
funnel containing dichloromethane and water. The organic layer was
3
, 10 K) 2931 m,
2
−
1
cm (Figure 3); UV/vis (N
10 K) |D/hc| = major: 0.627 minor: 0.651 cm , |E/hc| = major: 0.0073
, 10 K) λmax 350 nm (Figure 5); EPR (Ar,
2
−1
−1
minor: 0.0048 cm ; Z major: 3240 minor: 3525, X major: 5700
1
2
G
DOI: 10.1021/jacs.6b07444
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
minor: 5885, Y major: 6008 minor: 6092, Z major: 10200 minor:
(10) Lee, T. J.; Taylor, P. R. Int. J. Quantum Chem. 1989, 36, 199−
2
2
1
0400 G; microwave frequency 9.713 GHz (Figure 6).
Pent-1-en-3-yne (4). Triethylamine (180 mL) was degassed
207.
4
2
(11) Lee, T. J. Chem. Phys. Lett. 2003, 372, 362−367.
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(
nitrogen, 30 min) in a thick-wall pressure tube, and to it was added
vinyl bromide (6.0 mL, 85.3 mmol), copper iodide (0.59 g, 3.1 mmol),
and bis(triphenylphosphine) palladium(II) dichloride (0.997g, 1.4
mmol). In a separate flask, propyne (8 mL, 106 mmol) was condensed
at −78 °C and added via syringe at −78 °C. The pressure tube was
sealed with a Teflon screw cap, the cold bath removed, and the
mixture stirred at ambient temperature for 18 h. The mixture was first
purified by simple distillation. The resulting distillate was then
fractionally distilled twice to afford pent-1-en-3-yne (4) with traces of
triethylamine. To isolate the pure enyne in deposition glassware, a
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2
(
deposition tube was cooled to −78 °C in liquid N under vacuum, and
2
the sample was opened to the system. The resulting IR spectrum did
(
1
not show the presence of triethylamine. H NMR (400 MHz, CDCl ):
3
δ 5.76 (ddq, J = 17.5, 11.0, 2.3 Hz, 1H), 5.55 (dd, J = 17.5, 2.2 Hz,
(
1
1
H), 5.38 (dd, J = 11.0, 2.2 Hz, 1H), 1.95 (d, J = 2.3 Hz, 3H). IR (N2,
0 K) 2976 s, 2925 m, 2862 w, 2341 w, 2241 s, 1848 w, 1716 w, 1613
Thomas, P. S.; Simmons, C. S.; Stanton, J. F.; McMahon, R. J. J. Am.
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−1
9
82 s, 929 s, 680 w cm (Figure 4).
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(
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b07444.
■
́
*
S
2002, 124, 14716−14720.
(23) Stanton, J. F. J. Chem. Phys. 2007, 126, 134309.
(24) Nimlos, M. R.; Davico, G.; Geise, C. M.; Wenthold, P. G.;
Lineberger, W. C.; Blanksby, S. J.; Hadad, C. M.; Petersson, G. A.;
Experimental and computational details; H and 13C
NMR spectra; computed energies, Cartesian coordinates,
harmonic vibrational frequencies, and infrared intensities
of all species studied at all levels of theory (PDF)
1
Ellison, G. B. J. Chem. Phys. 2002, 117, 4323−4339.
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AUTHOR INFORMATION
Corresponding Author
mcmahon@chem.wisc.edu
■
Rao, K. N., Ed.; Academic Press: Orlando, 1985; Vol. III, pp 321−419.
(28) Winnewisser, M.; Winnewisser, B. P.; Medvedev, I. R.; De Lucia,
*
F. C.; Ross, S. C.; Bates, L. M. J. Mol. Struct. 2006, 798, 1−26.
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Notes
The authors declare no competing financial interest.
(
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ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the National
Science Foundation (CHE-1011959 and CHE-1362264). We
also acknowledge NSF support for Departmental EPR
instrumentation (CHE-0741901) and computing facilities
(
(
Chem. Chem. Phys. 2006, 8, 5304−5316.
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DOI: 10.1021/jacs.6b07444
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