Photooxidation of Trimethyl Phosphite in Nitrogen, Oxygen, and para-Hydrogen Matrixes at Low Temperatures

Article abstract of DOI:10.1021/acs.jpca.6b12296

(Chemical Equation Presented) Trimethyl phosphite (TMPhite) was photooxidized to trimethyl phosphate (TMP) in N₂, O₂, and para-H₂ matrixes at low temperatures to correlate the conformational landscape of these two molecules. The photooxidation produced the trans (TGG)-rich conformer with respect to the ground state gauche (GGG) conformer of TMP in N₂ and O₂ matrixes, which has diverged from the conformational composition of freshly deposited pure TMP in the low-temperature matrixes. The enrichment of the trans conformer in preference to the gauche conformer of TMP during photooxidation is due to the TMPhite precursor, which exists exclusively in the trans conformer. Interestingly, whereas the photooxidized TMP molecule suffers site effects possibly due to the local asymmetry in N₂ and O₂ matrixes, in the para-H₂ matrix owing to the quantum crystal nature the site effects were observed to be self-repaired.

Full text of DOI:10.1021/acs.jpca.6b12296

Article
pubs.acs.org/JPCA
Photooxidation of Trimethyl Phosphite in Nitrogen, Oxygen, and
para-Hydrogen Matrixes at Low Temperatures
N. Ramanathan,† K. Sundararajan,*,†,‡ R. Gopi,†,‡ and K. Sankaran†,‡
†Materials Chemistry and Metal Fuel Cycle Group and ‡Homi Bhabha National Institute, Indira Gandhi Centre for Atomic Research,
Kalpakkam-603102, India
S
* Supporting Information
ABSTRACT: Trimethyl phosphite (TMPhite) was photooxidized to trimethyl phosphate (TMP) in N2, O2, and para-H2
matrixes at low temperatures to correlate the conformational landscape of these two molecules. The photooxidation produced
the trans (TGG)-rich conformer with respect to the ground state gauche (GGG) conformer of TMP in N2 and O2 matrixes,
which has diverged from the conformational composition of freshly deposited pure TMP in the low-temperature matrixes. The
enrichment of the trans conformer in preference to the gauche conformer of TMP during photooxidation is due to the TMPhite
precursor, which exists exclusively in the trans conformer. Interestingly, whereas the photooxidized TMP molecule suffers site
effects possibly due to the local asymmetry in N2 and O2 matrixes, in the para-H2 matrix owing to the quantum crystal nature the
site effects were observed to be self-repaired.
phosphates are attractive candidates, where geminal and vicinal
hyperconjugation interactions together contribute to their
stability. In addition to the electronegativity difference between
the phosphorus and oxygen atoms, the presence of a vacant d
orbital in the phosphorus atom is a notable contributor to such a
geminal interaction. The conformers of alkoxyalkanes (acetals
and ketals) and organic carbonates do not earn this extra geminal
stabilization because the electronegativity difference between
carbon and oxygen is smaller and because of the nonavailability
of the vacant d orbital on carbon.15 Hence, compared to carbon
compounds, the compounds of phosphorus are very special
because the phosphorus−oxygen linkage enjoys this extra
geminal stabilization.
The study of conformations of phosphates is important
because tri-n-butyl phosphate (TBP) is used as an extractant in
the solvent extraction process and TBP is considered to be the
workhorse of the nuclear reprocessing industry.16 The large
multitude of conformations for TBP is due to the presence of
conformationally flexible butyl groups and its bonding to oxygen.
In reprocessing, the metal−TBP complexes are extracted into the
1. INTRODUCTION
The study of conformations of molecules with O−P−O and O
P−O connectivity assumes significance because these are the
linkages that decide the structures of biologically important
phosphate molecules.1,2 Energy ordering in different conforma-
tional isomers is decided by stabilizing electron delocalization
(hyperconjugative) and destabilizing steric interactions.3−6
Although steric interactions keep the bulky groups away from
each other, the hyperconjugative delocalization interactions
often dictate that the sterically less favorable orientation is an
energetically favorable one.7−12 The conformations of dime-
thoxy methane (DMM) can be considered to be the classic
example in which the sterically crowded gauche conformer
turned out to be the ground-state conformer rather than the trans
conformer because of the hyperconjugative interaction (the lone
pair on oxygen to antibonding σ*(C−O) delocalization
interaction) in the Gauche conformer.13 Because three or four
atoms with a lone pair of electrons are required to observe these
interactions, these are pointed out as 1,4 or vicinal (four-
centered) interactions. Yet another hyperconjugative interaction,
referred to as 1,3 or geminal (three centered) interactions, is
normally operative in conformers with two or three atoms with a
lone pair of electrons that differ significantly in electro-
negativity.14 Phosphorus compounds such as phosphites and
Received: December 7, 2016
Revised: February 24, 2017
Published: February 24, 2017
© XXXX American Chemical Society
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organic phase; consequently, the electron density of the
phosphoryl group is substantially reduced. With regard to the
conformational orientation of the phosphate, the question arises
as to whether metal complexation affects the conformational
preferences in phosphates. This question can best be answered
by analyzing the conformational energy ordering of phosphites,
which lack the phosphoryl group and are considered to be an
appropriate representative to model the effect of the phosphoryl
group as a result of metal complexation. The simple and
straightforward phosphite and phosphate model would then be
trimethyl phosphite (TMPhite, (OCH3)3P) and trimethyl
phosphate (TMP, (OCH3)3PO), respectively. The exper-
imental and computational studies on the conformations of
TMPhite have been reported.17 Through DFT computations,
the energy ordering of the different conformers of TMPhite was
found to follow the sequence C1(TG G , 0.00 kcal/mol) <
Cs(TG+G−, 1.22 kcal/mol) < C1(TTG −, 1.38 kcal/mol) <
C3(G G G −, 2.46 kcal/mol) (T and G refer to trans and
gauche orientations of methyl groups with respect to the
phosphorus lone pair, represent the clockwise/anticlockwise
orientation of methyl groups with respect to the phosphorus lone
pair, and Cn refers to the symmetry of the conformer).18 The
dihedral angles of conformers TMPhite and TMP with
appropriate reference atoms/groups are given in Table S1 of
the Supporting Information. Through effusive (ambient 298 K
and high temperature 410 K) and supersonic beam experiments
(∼100 K), it was confirmed that only the ground state
C1(TG G ) conformer of TMPhite was populated at 12 K.
During deposition at 12 K, the higher-energy Cs(TG+G−) and
C1(TTG ) conformers of TMPhite interconverted and ex-
clusively produced the ground-state C1(TG G ) conformer.
The barriers for conformer interconversion from higher-energy
Cs(TG+G−) and C1(TTG ) to ground-state C1(TG G )
conformer of TMPhite were calculated to be 0.1 and 0.8 kcal/
mol, respectively. These small barriers are responsible for higher-
energy conformers interconverting to the ground-state con-
former of TMPhite during deposition. This process, known as
conformational cooling, has been reported well in the
literature.19−21 Because the C3(G G G ) conformer is placed
at a relatively high energy of ∼2.5 kcal/mol with respect to the
ground-state conformer, this conformer did not presume any
experimental significance. Interestingly, for TMP in the presence
of an extra phosphoryl group in comparison to TMPhite, DFT
computations disclosed that the conformer with C3(G G G )
geometry, the one that is present as the highest-energy
conformer in TMPhite, corresponds to the ground state and
the ground-state C1(TG G ) conformer of TMPhite occurs as
the first higher-energy conformer. Unlike TMPhite with the
existence of the C1(TG G ) conformer experimentally, for
TMP, in addition to the ground-state C3(G G G ) conformer,
the higher-energy C1(TG G ) conformer (ΔE ≈ 0.8 kcal/mol)
has significant population in the low-temperature matrix.22−24
The barrier at the B3LYP/6-311++G(d,p) level of theory for the
interconversion of the higher-energy C1(TG G ) conformer to
the ground-state C3(G G G ) conformer was calculated to be
1.6 kcal/mol.
isolated conditions at low temperatures in order to probe the
conformational landscape of both molecules. Because the
conformational preferences of both the molecules are very
different, it will be intriguing to examine whether the intrinsic
conformational behavior of TMPhite is conserved as a result of
the cage effect of the matrix. If oxidation is achieved in its ground-
state C1(TG G ) conformer of TMPhite, then an adiabatic
transformation to the higher-energy C1(TG G ) conformer of
TMP will be accomplished. If the matrix, because of its cage
effect, preserves this conformer, then the photooxidation of
TMPhite can be expected to yield only the higher-energy
C1(TG G ) conformer of TMP. It will also be interesting to
investigate whether any relaxation of the photooxidation product
of TMPhite to the ground-state C3(G G G ) conformer of
TMP occurs in the matrix.
Until recently, the inert (Ne/Ar/Kr/Xe) and diatomic (N2/
O2) gases were considered only as possible matrixes for matrix
isolation experiments. A decade or two ago, para-H2 (p-H2) gas
emerged as an alternate and attractive matrix, and it has proved
its supremacy compared to conventional inert matrixes.26−35
Because of the small mass and large-amplitude zero-point
vibration, p-H2 is being referred to as a quantum crystal. The
quantum crystal nature of solid p-H2 makes it an attractive matrix
gas for matrix isolation of guest molecules. The sharp line width
in solid p-H2 in comparison to conventional matrixes is one of its
exceptional properties. In addition, the p-H2 crystal provides a
homogeneous and interaction-free environment for guest
molecules embedded in it due to the J = 0 rotational wave
function, which is completely spherical and symmetric, unlike
ortho-H2 (o-H2). The solid p-H2 crystallizes in a pure hexagonal
close-packed (hcp) lattice, contrary to other inert and diatomic
matrixes, making the optical spectra simple and sharp. The
intrinsically restless nature of p-H2 even at absolute zero
temperature owing to the large zero-point energy causes multiple
trapping sites and crystal defects around the guest molecules to
be self-repaired.28 In addition, the large lattice constant of solid p-
H2 causes the interaction to be weaker between the guest and the
host molecules, and as a result, the lifetime of the excited states of
the guest molecules becomes longer in solid p-H2.30 The unique
properties compared to conventional matrixes make p-H2 a
promising matrix for studying the molecular properties at low
temperatures, which can mimic the gas-phase behavior. The
insignificant or diminished cage effect is also an attractive
property for p-H2 for studying the photoproducts including
radicals under isolated conditions.36−39
The scope of the present study is therefore twofold. The first is
to perform the photooxidation of TMPhite at low temperature in
O2-doped N2 and pure O2 matrixes to establish a correlation
between the conformers of TMPhite and TMP. The second is to
accomplish the photooxidation of TMPhite in an O2-doped p-H2
matrix and compare the results with O2-doped N2 in pure O2
matrixes.
2. EXPERIMENTAL AND COMPUTATIONAL DETAILS
TMPhite (Merck, >98%) and TMP (Merck, >98%) were used
without further purification. The samples were, however,
subjected to several freeze−pump−thaw cycles before perform-
ing the experiments. Nitrogen (Inox, purity 99.9995%) and
oxygen (Praxair, purity 99.999%) were used as matrix gases. O2
was used as a dopant gas for photooxidation experiments. For
isotopic scrambling experiments, 18O2 (Aldrich; 99 atom %) gas
was used. Photoirradiation of the matrix (for about 30 min) was
performed using a 1600 W broad band ozone-free high-pressure
Reva et al. have reported the photooxidation of trivalent
phosphorus (triphenyl phosphine) to pentavalent phosphorus
(triphenyl phosphine oxide) at low temperatures.25 Similarly,
phosphite (trivalent phosphorus) can be thought to be
photooxidized to phosphate (pentavalent phosphorus) in the
presence of oxygen; it will be interesting to perform this
experiment to produce TMP in situ from TMPhite under
B
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Figure 1. Correlation diagram of the conformations of TMPhite (left) and TMP (right). The ZPE-corrected relative energies in kcal/mol computed at
B3LYP level of theory with the 6-311++G(d,p) basis set are shown alongside the conformers.
xenon arc lamp source (model 201-1K air-cooled light source,
Sciencetech Inc., Canada). Photoirradiation experiments were
carried out on TMPhite by doping 5% O2 in N2 and p-H2
matrixes. The photolysis experiment was also conducted on
TMPhite in a pure O2 matrix. TMPhite along with the
appropriate gases were mixed in a 1 L stainless steel vacuum
chamber and subsequently deposited onto a cold KBr substrate
maintained at 12 K. For p-H2, the deposition was accomplished at
2.5 K. The typical concentration of TMPhite to matrix was
1:1000.
The infrared spectra of the matrix-isolated samples were
recorded in the range of 5000 to 400 cm−1 using a Bruker Vertex
C
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Table 1. Relative Energies, Population, and Degeneracies for the Different Conformers of TMPhite and TMP Calculated at the
B3LYP and CCSD Levels of Theory with the 6-311++G(d,p) Basis Set
a
b
B3LYP/6-311++G(d,p)
CCSD/6-311++G(d,p)
conformers
degeneracy
relative energy (kcal/mol)
population (%)
TMPhite
relative energy (kcal/mol)
population (%)
C1(TG G )
Cs(TG+G‑)
C1(TTG )
C3(G G G )
6
3
6
2
0.00
1.22
1.38
2.46
85.7
5.5
0.00
1.71
1.37
3.29
88.7
2.5
8.4
8.7
0.4
0.1
TMP
C3(G G G )
C1(TG G )
Cs(TG+G‑)
3
6
0.00
0.82
2.26
56.5
41.6
1.88
0.00
0.45
2.47
41.2
57.8
1.0
3
a
b
Corrected for zero-point energy. Single-point energy. (The optimized structure at the B3LYP level of theory was used.)
Figure 2. Infrared spectra of matrix-isolated TMP (grid A: PO stretching region) and TMPhite (grid B: P−O stretching region) in N2 and p-H2
matrixes. Grid A: (a) TMP/N2 at 12 K and (b) TMP/p-H2 at 2.5 K. Grid B: (a) TMPhite/N2 at 12 K and (b) TMPhite/p-H2 at 2.5 K. The typical
concentration of TMPhite or TMP to the matrix is 1:1000.
70 FTIR spectrometer with a resolution of 0.5 cm−1 using a
3. RESULTS AND DISCUSSION
liquid-N2-cooled mercury cadmium telluride (MCT) detector.
A correlation diagram indicating the conformational isomers of
Before and after photoirradiation, the infrared spectra were
recorded at 12 K (N2 and O2 matrixes) and 2.5 K (p-H2 matrix).
Subsequently, the matrix was warmed to ∼30 K for N2 and O2
matrixes and to 4.5 K for p-H2, kept at this temperature for 15
min, and recooled to ∼12 K (N2 and O2 matrixes) and 2.5 K (p-
H2 matrix). The spectra of the matrix after annealing were again
recorded.
p-H2 gas was produced by passing n-H2 through the ortho/
para converter at low temperatures. The details of the
experimental setup and the preparation of pure p-H2 gas are
described elsewhere.40
DFT computations were used for the optimization of the
conformers of TMPhite and TMP. Geometry optimization was
performed using a B3LYP hybrid-exchange correlation func-
tional using the 6-311++G(d,p) basis set. Single-point
calculations were performed using the CCSD level of theory
with the aforementioned basis set. All computations were carried
out using a Gaussian 09 package.41
TMPhite and TMP together with their ZPE-corrected relative
energies is shown in Figure 1. The computed relative energies
together with the room-temperature population of the different
conformers of TMPhite and TMP and their degeneracies are
presented in Table 1. Figure 2 (grid A) shows the infrared
spectral features in the PO stretching region of TMP in N2 and
p-H2 matrixes. Among the PO, P−O, and O−C vibrational
spectral regions, the PO stretching region of TMP shows the
maximum vibrational wavenumber separation of the two
conformers, and hence this region alone is presented in Figure
2. The vibrational wavenumbers that occur at 1286.7/1283.7 and
1306.5/1301.7 cm−1 in N2 and at 1288.2/1283.9 and 1308.5/
1304.2 cm−1 in p-H2 matrixes correspond to ground-state
C3(G G G ) and higher-energy C1(TG G ) conformers,
respectively.22,23,40 Because TMPhite does not possess any
characteristic phosphoryl group, the P−O and O−C stretching
regions are used to obtain its conformational signature. Figure 2,
grid B, shows the P−O spectral region of TMPhite. It can be
reiterated that through systematic effusive and supersonic beam
experiments the ground-state C1(TG G ) conformer was alone
D
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Table 2. Comparison of Computed Vibrational Wavenumbers of the C1(TG G ) Conformer of TMPhite, C3(G G G ) and
a
C1(TG G ) Conformers of TMP with Experimental Wavenumbers in N2, O2, and p-H2 Matrixes
experimental vibrational wavenumber (cm−1
)
computed vibrational wavenumber
b
conformers
C1(TG G )
(cm−1
)
N2 matrix
TMPhite
731.4, 736.7, 742.2
O2 matrix
p-H2 matrix
mode assignments
687.8 (186)
730.3 (155)
745.7 (67)
733.3, 733.8
732.8
P−O stretching
760.1
751.7, 759.9, 763.8
775.4, 787.9
761.4
777.8
775.8, 780.6
1022.2, 1025.1
1030.8, 1035.0
1066.8
1030.8 (333)
1047.2 (310)
1076.5 (89)
1019.0, 1025.4, 1029.3 1025.3, 1031.2
O−C stretching
1036.9
1068.4
1038.9
1070.3
TMP
d
16
C3(G G G )
C1(TG G )
1261.2 (213)
1225.7 (187)
1290.1 (265)
1251.6 (271)
1286.7
1285.2
P O stretching
c
cd
,
18
1248.0 (−38.7)
1301.7, 1306.5
1247.1(−38.1)
P Ostretching
d
16
1309.7
P O stretching
c
c
cd
,
18
1262.8 (−38.9) , 1267.2 (−39.3)
1273.2 (−36.5)
P O stretching
a
b
The computations have been performed at the B3LYP level of theory with the 6-311++G(d,p) basis set. Intensities in km mol−1 are given in
c
d
18
16
parentheses The experimental shift of the P O stretch with respect to the P O stretch is given in parentheses Wavenumbers for TMP in 5%
O2 doped in the p-H2 matrix.
Figure 3. Infrared spectra of oxygen-doped TMPhite in the N2 matrix (grid A) and conformational yield comparison (grid B) at 12 K in the PO
spectral region of TMP. Grid A: (a) TMPhite/N2 with 5% 16O2 before photolysis at 12 K. (b) Same as in a but photolyzed for 30 min. (c) TMPhite/N2
with 2.5% 16O2 and 2.5% 18O2 after photolysis at 12 K. (d) TMP/N2 with 5% 16O2 at 12 K. (e) TMPhite/N2 with 5% 18O2 after photolysis at 12 K (inset).
The wavenumbers marked in black correspond to the spectral features that exactly match with pure TMP. The wavenumbers marked in pink are due to
the additional features that are produced as a result of the effect of photolysis. Grid B: Blue-[line + symbol], infrared spectra of TMP produced through
the photooxidation of TMPhite in the N2 matrix; red-[line], pure TMP/N2 with 5% 16O2 at 12 K. The typical concentration of TMPhite or TMP to the
matrix is 1:1000.
identified, and all of the spectral features observed in the low-
temperature matrix correspond to this conformer. NBO analysis
revealed that the presence of the phosphoryl group in TMP
induces a geminal hyperconjugation interaction and causes the
C3(G G G ) conformer to be energetically more favorable
compared to the C1(TG G ) conformer.17 Because of the
absence of the phosphoryl group in TMPhite, the contribution to
stability through geminal interaction is appreciably reduced in
the C3(G G G ) conformer; consequently, this conformer is
placed highest in the energy ladder. The computed vibrational
wavenumbers of the conformers of TMPhite and the
experimental wavenumbers are presented in Table 2.
3.1. Photooxidation of TMPhite in N2 and O2 Matrixes.
TMPhite was deposited in N2 matrix together with O2 dopant
and photolyzed using a broad-band xenon source in an attempt
to oxidize TMPhite. Trace a in Figure 3 (grid A) shows the
infrared spectrum of matrix-isolated TMPhite with 5% 16O2 in an
N2 matrix at 12 K prior to photoirradiation, and trace b presents
the spectrum recorded after the photolysis of the above mixture
for 30 min. Trace c is the infrared spectrum corresponding to
photoirradiation experiments where O2 is isotopically scrambled
with 2.5% 16O2 and 2.5% 18O2. Trace d is the infrared spectrum of
matrix-isolated pure TMP in an N2 matrix with 5% O2. The
features observed at 1286.7 and 1301.7/1306.5 cm−1 in trace d
are due to the PO stretch in the C3(G G G ) and
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C1(TG G ) conformers of TMP in the N2 matrix.22,23 It is clear
from the figure that upon photolysis TMPhite is oxidized to
TMP as new vibrational features characteristic of the PO
stretching region of TMP are produced. The photolysis
experiments of TMPhite in the presence of O2 yielded features
at 1305.9, 1301.7, 1295.5, 1290.5, 1286.7, 1283.9, and 1276.8
cm−1 at 12 K. The comparison of the vibrational features of pure
TMP in the O2-doped N2 matrix disclosed that the features
characteristic of both C3(G G G ) and C1(TG G ) con-
formers of TMP are produced in the matrix during the
photooxidation of TMPhite. The strong, broad doublet feature
at 1305.9/1301.7 cm−1 relative to the weak feature at 1286.7
cm−1 in the photooxidation experiments (Figure 3, grid B)
clearly revealed the dominant presence of the C1(TG G )
conformer of TMP produced from the C1(TG G ) TMPhite
precursor in preference to the C3(G G G ) conformer. Of
course, there is a slight shift in the higher-wavenumber
component of TMP produced through the photolytic route
(1305.9 cm−1 is red shifted by 0.6 cm−1 with respect to 1306.5
cm−1 for pure TMP). It is also obvious from Figure 3 that the
relative intensity of the C1(TG G ) conformer (1305.9/1301.7
cm−1) with respect to the C3(G G G ) conformer (1283.9/
1276.8 cm−1) of TMP formed from photolysis is not identical to
that of pure TMP.
than for the C3(G G G ) conformer, which confirms the
dominant production of the C1(TG G ) conformer compared
to the C3 (G G G ) and is consistent with the observation made
in the PO stretching region. The C−O vibrational region of
TMP, produced through photolysis, was broadened because the
region occurs very close to the parent TMPhite absorption
(Figure S1 in Supporting Information) and therefore is not
considered in analyzing the conformational population.
In the photooxidation experiments, in addition to the
generation of TMP in situ under matrix-isolated conditions,
42,43
with small intensity, the spectral features of O3
and N2O44
were also observed (not shown in the figure), which clearly
suggest that oxygen atoms are indeed produced in the matrix
during photolysis and that they react with O2 and N2 to form the
aforementioned products. Of course, the generation of TMP
from TMPhite itself confirms the production of oxygen atoms in
the N2 matrix. Therefore, to identify the source of oxygen atoms,
blank experiments were carried out by photolyzing the O2-doped
N2 matrix without TMPhite. This exercise produced neither O3
nor N2O in the N2 matrix, probably indicating that TMPhite
alone is the light-absorbing chromophore in the matrix. The
nonformation of O3 in the N2 matrix is due to the ozone-free
xenon arc lamp used in the photolysis experiments, which has
negligible intensity in the 200−250 nm region (output 0.31%). It
has been reported by Schriver-Mazzuoli et al. that O3 is produced
in the wavelength range of 245−266 nm.45 The UV−visible
absorption spectrum of pure TMPhite was recorded, and the
maximum absorption was observed at 232 nm with tailing
beyond 250 nm (Figure S5 in Supporting Information). The
absorption beyond 250 nm could possibly be responsible for
TMPhite behaving as a light-absorbing species in the photo-
oxidation experiments.
In addition to the PO spectral region, the P−O vibrational
region is also considered to investigate the conformational
composition of TMP as a result of the photooxidation of
TMPhite. Figure 4 shows a comparison of photooxidized
The photooxidation experiments unambiguously revealed that
the photochemical addition of oxygen to TMPhite does yield
predominantly the higher-energy C1(TG G ) conformer. The
population of the C1(TG G ) conformer of TMP thus produced
via photooxidation is more than the as-deposited pure TMP in
the O2-doped N2 matrix, and the nonequilibrium population is
therefore achieved in the matrix.
Because TMPhite exists only in the ground-state C1(TG G )
conformer, one would expect to form the C1(TG G )
conformer of TMP exclusively in the photolysis experiment,
and to our surprise, a small amount of the C3(G G G )
conformer of TMP was also observed. The formation of the
C3(G G G ) conformer of TMP could be due to the localized
heating of the matrix during photoirradiation. During photolysis,
the TMPhite molecule is electronically excited through photon
absorption, and the excess energy gets dissipated to the
surrounding matrix, which causes the localized heating of the
matrix. As a result, the C3(G G G ) conformer is produced at
the expense of the freshly formed C1(TG G ) conformer of
TMP from TMPhite. Because the C3(G G G ) conformer of
TMP is energetically favorable in the potential energy surface, the
route by which it is formed at low temperatures is through the
energy dissipation in the matrix from the higher-energy
C1(TG G ) conformer. Furthermore, the operation of the
nonequilibration of excited rotamers (NEER) principle needs to
be considered in the photoprocess.46,47 As a direct consequence
of NEER, the photooxidation of TMPhite is expected to produce
TMP, which is predominately enriched in the C1(TG G )
conformation. The fact that a small amount of the C3(G G G )
conformer of TMP is also produced in addition to the
C1(TG G ) conformer should be due to conformational cooling
Figure 4. Infrared spectra of oxygen-doped TMPhite in the N2 matrix in
the P−O spectral region of TMP. (a) TMPhite/N2 with 5% 16O2 before
photolysis at 12 K. (b) Same as in a but photolyzed for 30 min. (c)
TMP/N2 with 5% 16O2 at 12 K.
TMPhite and pure TMP in the O2-doped N2 matrix in the P−
O stretching region. The feature that occurs at 866.2 cm−1 was
assigned to the C3(G G G ) conformer, whereas the other two
features observed at 862.1 and 850.5 cm−1 were attributed to the
C1(TG G ) conformer of TMP in the N2 matrix.22,23 It is
therefore clear that the photooxidation produces features
characteristic of the C1(TG G ) conformer at a higher intensity
F
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Figure 5. Comparison of photoirradiation experiments of oxygen-doped TMPhite in p-H2 (grid A) and conformational yield comparison (grid B) in the
PO spectral region of TMP. Grid A: Infrared spectra of (a) TMPhite/p-H2 with 5% 16O2 before photolysis at 2.5 K. (b) Same as in a but photolyzed
for 30 min. (c) TMPhite/p-H2 with 2.5% 16O2 and 2.5% 18O2 after photolysis at 2.5 K. (d) TMP/p-H2 with 5% 16O2 at 2.5 K. Grid B: Blue-[line +
symbol], infrared spectra of TMP produced through the photooxidation of TMPhite in the p-H2 matrix; red-[line], pure TMP/p-H2 with 5% 16O2 at 2.5
K. The typical concentration of TMPhite or TMP to matrix is 1:1000.
that is occurring to a smaller extent by the light-induced local
heating of the matrix surrounding the molecule, which causes the
rigid matrix to become soft.
and P−O stretching regions are presented and the product
features of TMP are appropriately marked. To show the partial
oxidation of TMPhite in the O2-doped N2 matrix and complete
oxidation in the pure O2 matrix, in Figure S3 of the Supporting
Information the results of photolysis in the complete spectral
region are overlapped.
Clearly, the C3(G G G ) conformer of TMPhite is not
responsible for the production of the C3(G G G ) conformer of
TMP, and a conformer of this symmetry of TMPhite is the
highest-energy conformer in the energy ladder and has an
insignificant population in the matrix. It is evident from Figure 3
(grid A, trace c) that the vibrational features corresponding to
isotopic scrambling experiments (with mixtures of 16O2 and
18O2) produced additional features that are red-shifted by ∼35
It is apparent that the infrared spectra obtained from the
photooxidation experiment of TMPhite varies compared to the
spectra of pure TMP because the photolysis of TMPhite in the
O2-doped N2 matrix yielded additional features in the PO
stretching region at 1295.5, 1290.5, 1283.9, and 1276.8 cm−1 and
in the pure O2 matrix at 1229.2, 1292.7, and 1274.8 cm−1, which
makes the spectral region congested and complicates the
assignments. These additional features are not due to either
TMP or TMPhite, hence they could probably be due to the some
other photoproducts/adducts generated in the matrix. To check
for such a possibility, additional experiments were conducted in
the N2 matrix to examine whether the products observed were
due to the secondary products of TMP with oxygen as a result of
photoirradiation and/or due to the adducts of either TMP/
TMPhite and N2O and/or adducts of TMP/TMPhite and O3 in
an N2 matrix. These specific experiments did not produce the
characteristic set of additional features observed during the
photooxidation of TMPhite, and hence the assignments of these
features to the above-mentioned products can safely be ruled out.
Similarly, TMPhite can be expected to yield the photo-Arbuzov
product, dimethyl methyl phosphonate (DMMP), a reaction that
does not require oxygen because phosphites in solution when
exposed to UV irradiation were reported to undergo photo-
Arbuzov rearrangement.48 Prolonged irradiation of TMPhite in
an N2 matrix (in the absence of an oxygen dopant) did not
produce any characteristic vibrational features that could be
assigned to DMMP, which further confirms that these spectral
features are not due to the product of the photo-Arbuzov
18
cm−1, which correspond to the P O stretching of TMP
18
generated via photoirradiation. The experimental P O shift
16
with respect to P O stretching matches well with the
computed shift (at the B3LYP/6-311++G(d,p) level of theory)
for C3(G G G ) and C1(TG G ) conformers of TMP
respectively (Table 2). For TMP with 18O, in the PO region,
the experimental shift for the C3(G G G ) conformer was 38.7
cm−1 whereas the shift was 38.9 and 39.3 cm−1 for the
C1(TG G ) conformer in the N2 matrix with respect to TMP
with 16O.
The results of photooxidation experiments performed in the
pure O2 matrix are similar to what was observed in the case of the
N2 matrix. As a corollary to the N2 matrix, multiple features were
observed in the PO stretching region of TMP in the O2 matrix.
The infrared spectra of photoirradiation experiments performed
in the O2 matrix are shown in Figure S2 of the Supporting
Information. Notably, in the O2 matrix, the TMPhite precursor
was completely converted to TMP during photolysis, unlike in
the N2 matrix. It has been roughly estimated that in the N2 matrix
∼20% of initially deposited TMPhite is converted to TMP. In
addition to the PO stretching region, in the Supporting
Information (Figure S1 and S2) the results of photoirradiation
experiments in pure O2 and O2-doped N2 matrixes in the O−C
G
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Figure 6. Comparison of infrared spectra of TMPhite in (a) N2, (b) O2, and (c) p-H2 matrixes. (d) Calculated vibrational features of the ground-state
C1(TG G ) conformer of TMPhite is shown for comparison. Grids A and B correspond to O−C and P−O stretching regions of TMPhite, respectively.
The typical concentration of TMPhite to the matrix is 1:1000.
reaction. Probably, the matrix cage prevents the photo-Arbuzov
rearrangement of TMPhite to produce DMMP.
TMP exhibited isotopic (18O) shifts of 38.1 and 36.5 cm−1 with
respect to 16O, respectively, in the p-H2 matrix.
If these additional features are not due to adducts or any other
ancillary products, then subsequent probing is imperative to sort
out to which group these features actually belong. To address this
issue, experiments were performed in the p-H2 matrix. Among
the matrixes explored so far, the p-H2 matrix host is inherently
superior because of its condensed-phase properties, which
provide a minimally perturbing environment for the guest
molecule with a reduced or diminished cage effect. Therefore,
photooxidation experiments were conducted in the p-H2 matrix
to envisage what difference p-H2 actually makes in comparison to
the conventional N2 and O2 matrixes.
3.2. Photooxidation of TMPhite in the p-H2 Matrix. At
the outset, the p-H2 produced was examined for its purity by low-
temperature infrared measurements.40 When p-H2 was directly
deposited upon its production from the p-H2 generator, the
purity was estimated to be ∼99.3% (∼0.7% o-H2). For
experiments with TMPhite/TMP with p-H2 gas, the o-H2
concentration was found to be ∼1.5%. When experiments were
performed with a 5% O2 dopant, the paramagnetic O2 increased
the concentration of the o-H2 impurity to ∼7%. In essence, the
photooxidation experiments could be performed only with a p-
H2 purity of ∼93%. However, it was observed that at this higher
level of o-H2 impurity, additional splitting of the spectral features
of both TMPhite and TMP was not observed.
In other words, the secondary features, those obtained in the
N2 and O2 matrixes, were not observed in the p-H2 matrix.
Furthermore, the spectrum of TMP produced through the
photolytic route is very simple in contrast to those of the N2 and
O2 matrixes. Therefore, the photooxidation experiments
performed in the p-H2 matrix clearly delineate that the product
is only TMP and no other secondary products are formed. It is
important to point out that the vibrational features of TMP in the
5% O2-doped p-H2 matrix exhibited a slight blue shift of 2 cm−1
with respect to pure TMP in the p-H2 matrix. In the Supporting
Information (Figure S4), the results of photoirradiation
experiments in the O2-doped p-H2 matrix in the O−C and P−
O stretching regions are presented.
To rationalize the experimental observation in the p-H2 matrix,
the infrared spectra of TMPhite, the photoprecursor, was
critically examined. It is clear from the O−C (Figure 6, grid A)
and P−O (Figure 6, grid B) stretching regions of TMPhite that
the infrared spectrum in the p-H2 matrix is free from multiple site
effects, unlike in the N2 and O2 matrixes. The computed
spectrum of the C1(TG G ) conformer of TMPhite (Figure 6,
trace d) is given for comparison. It can be recalled that for
TMPhite the ground-state C1(TG G ) conformer alone is
populated in low-temperature matrixes. Clearly the simple
spectrum, free from site effects, is attributable to the quantum
crystal nature of p-H2, and conventional N2 and O2 matrixes
suffer severely because of the site effect. Because of the tight cage
effect of the N2 and O2 matrixes, these site-split features of
TMPhite are retained after its photooxidation to TMP and are
manifested as additional features in the experiment. Certainly,
the cage surrounding the TMP produced from TMPhite
photooxidation is not identical to the cage formed when pure
TMP vapor was immediately deposited onto the matrix. It is
expected that annealing can transform all different site-split
When photoirradiation was performed under identical
conditions on TMPhite with 5% 16O2 in a p-H2 matrix at 2.5
16
K, surprisingly, vibrational features in the P O stretching
region of TMP were obtained without any spectral congestion
(Figure 5, grid A, traces b and d). The experiments performed
using 2.5% 16O2 and 2.5% 18O2 in the p-H2 matrix produced
vibrational features characteristic of an isotopic shift correspond-
18
ing to the P O stretching region of TMP (Figure 5, grid A,
trace c). The C3(G G G ) and C1(TG G ) conformers of
H
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Figure 7. Effect of annealing on the photolyzed sample of 5% O2-doped TMPhite. Grid A (N2 matrix): (a) as deposited and photoirradiated at 12 K and
(b) the annealed sample at 30 K after photoirradiation. Grid B (p-H2 matrix): (c) as deposited and photoirradiated at 2.5 K and (d) the annealed sample
at 4.5 K after photoirradiation.
features irreversibly to the stable ones, but surprisingly, on the
annealing of the N2 matrix, the features at 1301.7 and 1295.5
cm−1 widened and merged to form a broad feature centered at
1298.5 cm−1 in comparison to the photooxidized sample (Figure
7, grid A, trace b). In addition, the weak shoulders observed at
1290.5/1283.9 cm−1 are shifted to 1291.4/1282.9 cm−1 with the
increase in intensity on annealing. This observation implies that
the cage effect is still operative under the conditions of annealing,
whereas in the p-H2 matrix the annealing to the highest possible
temperature of 4.5 K does not change the intensity of the features
produced through photooxidation (Figure 7, grid B). Another
striking difference between the conventional N2 and O2 matrixes
and the p-H2 matrix is the composition of C3(G G G ) and
C1(TG G ) conformers of TMP formed. Whereas C1(TG G )-
rich TMP was produced in N2 and O2 matrixes, the proportion of
the C1(TG G ) conformer is much less in the p-H2 matrix, and
the near-equilibrium population was achieved in comparison to
pure TMP in the p-H2 matrix (Figure 5, grid B). The intrinsically
restless nature of the p-H2 molecule with a large-amplitude zero-
point vibration and a large lattice constant is likely responsible for
achieving the near-equilibrium population.
Nevertheless, both TMPhite and TMP are expected to be
structurally alike with the similar conformational landscape
except for the presence of extra oxygen in TMP, the unique
stereoelectronic factors were observed to alter the individual
conformational preferences. The presence of phosphoryl group
has a profound influence on the conformational control of TMP
and in turn, the geminal hyperconjugative charge-transfer
delocalization interactions regulate the conformational space of
both TMPhite and TMP.
Another significant outcome of this study is the interesting
variation in photolytic behavior observed in the case of p-H2. In
the conventional N2 and O2 matrixes, multiple spectral features
during the photooxidation of TMPhite were observed. The
multiple features on photooxidation are simply the manifes-
tations of site-splitting of the ground-state C1(TG G ) con-
former of TMPhite. The site splitting of the photoproduct was
observed to be preserved on annealing because of the cage effect
of N2 and O2 matrixes. Consequently, this work highlights that
among the matrixes, p-H2 is superior as site effects on either the
precursor or the photoproduct were found to be automatically
repaired as a result of its quantum crystal nature. Furthermore, in
the p-H2 matrix, the near-equilibrium or equilibrium conforma-
tional population was achieved because TMP/TMPhite
molecules embedded in the p-H2 matrix can be considered to
be conformationally labile.
4. CONCLUSIONS
In this work, the photooxidation of TMPhite in N2, O2, and p-H2
matrixes was studied. When TMP was produced in situ in low-
temperature matrixes through the photolytic pathway, the
nascent population distribution of C3(G G G ) and
C1(TG G ) conformers was considerably affected. In compar-
ison to the conformational composition of pure TMP,
photooxidation produced a preponderantly enriched
C1(TG G ) conformer in the presence of a little C3(G G G )
conformer of TMP, thereby producing a nonequilibrium
conformational distribution at low temperatures. The presence
of the ground-state C1(TG G ) conformation of TMPhite, the
photolytic precursor, was attributed to the enrichment of the
C1(TG G ) conformer of TMP.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpca.6b12296.
Dihedral angles of TMP and TMPhite. Infrared spectra of
matrix-isolated TMPhite with an oxygen-doped N2 matrix
in the O−C and P−O stretching regions, of O2-doped
TMPhite before and after photolysis in the PO, O−C,
and P−O stretching regions in N2 and O2 matrixes, of
matrix-isolated TMPhite in the oxygen matrix, and of
oxygen-doped TMPhite in the p-H2 matrix in the O−C
I
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(18) The reason for keeping the phosphorus lone pair as a reference in
TMPhite is to have a common conformational terminology for both
TMPhite and TMP because the conformations are named by fixing the
phosphoryl group as a reference in TMP. Because the positions of the
lone pair in TMPhite and the phosphoryl group in TMP are identical,
the lone pair in TMPhite is taken as a reference. Furthermore, if a lone
pair is taken as a reference, then there will be commonality in the
conformations of TMPhite and TMP.
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and P−O stretching regions. UV−visible absorption
spectrum of pure TMPhite. (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: sundar@igcar.gov.in.
ORCID
■
K. Sundararajan: 0000-0002-9961-0030
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Prof. D. Mohan, Department of Chemical
Engineering, A. C. Tech Campus, Anna University, Chennai,
India, for providing computational support. R.G. gratefully
acknowledges the grant of a research fellowship from IGCAR,
Department of Atomic Energy, India. The authors thank the
anonymous reviewers for critically and meticulously analyzing
this work, which resulted in the constructive improvement of the
article.
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