1
0
O. Ünsalan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2013) xxx–xxx
Table 3 (continued)
Approximate description
PED
Symmetry
Calculated
Observed (Ar) b
Observed (Xe) b
m
I
m
I
m
I
d
s
s
as(CAC@C)
(C@C)
S
S
S
61[60] + S56[18] + S67[11]
33[58] + S31[30]
37[100]
B
A
A
61.7
54.5
34.9
0.2
0.5
0.01
s
(CAC)
a
See Table S1 (Supporting Information) for definition of symmetry coordinates;
m
, stretching; d, in-plane bending; w, wagging;
c
, out-of-plane bending;
s
, torsion; s,
ꢂ1
ꢂ1
symmetric; as, asymmetric; Ph, phenyl; n.obs., not observed; calculated wavenumbers (cm ) were scaled by 0.978; calculated infrared intensities in km mol ; PED’s
smaller than 10% are not given;.
b
Observed absorbances were normalized so that the total infrared intensity of the experimental bands (excluding those of the CH stretching bands) equals the corre-
sponding calculated intensity (for experimental CH stretchings intensities, the same normalizing factor obtained was applied.
conformation is clearly shown by the observation that its conver-
Table 4
sion to the most stable planar TS form requires the temperature
of the matrix to be increased (i.e., there is an energy barrier
separating the two forms, which must be surpassed during the
non-planar TS ? planar TS conversion). (b) On the other hand, a
detailed inspection of the spectra of the as-deposited matrices of
TS shows that bands due to the non-planar TS form can also be
observed in those spectra, thought as a very low intensity features.
Such observation requires that the non-planar TS form does exist
in the gas phase prior to deposition. Note that the amount of the
non-planar form in the as-deposited matrices of TS is only ꢁ5%, a
Calculated similarity distances (according to Eq. (1)) for calculated and observed non-
planar TS forms in reference to planar TS and CS.
Form a
d
TS
d
CS
5
1
2
2
3
6
9
0
0
0
3
3
3
3
6
6
6
–5 220.5
5–15
0–20
5–25
0–30
0–60
0–90
–30
306.4
211.7
181.6
170.4
164.2
228.9
201.7
179.2
184.9
215.6
201.8
197.6
188.7
210.5
181.3
201.3
191.4
284.3
241.7
220.8
204.3
344.4
373.8
236.0
208.6
253.4
276.7
251.8
268.7
318.8
307.2
360.5
277.8
–60
–90
value which would be compatible with a difference of energy
ꢂ1
between the non-planar and planar TS forms of ca. 9 kJ mol
.
0–60
0–90
0–120
0–150
0–90
0–120
0–150
However, this energy difference shall indeed be much smaller. In
fact, extensive conformational cooling during deposition of the
matrices can be expected to take place in the present case, since
the very low energy barrier separating the two forms can be easily
overcome at the time of landing of the TS molecules (carried on the
high temperature gaseous beam) onto the cold substrate of the
cryostat. Conformational cooling during cryogenic matrices depo-
sition is a common phenomenon and has been addressed in detail,
for instance, in Ref. [62].
The bold numbers indicate data for the form (30-30) which is proposed to corre-
spond to the non-planar observed form of trans-stilbene.
a
The structures are designated by the values of the two dihedral angles defined
around the two C–Phenyl bonds.
The conditions used to populate the non-planar TS form in the
present experiments (in situ photoisomerization of matrix-isolated
CS) appear to be particularly favorable for the stabilization of this
species. First of all, because the isomerization reaction takes place
under restricted volume conditions and, compared to planar TS,
the molecular volume of the non-planar TS form (the 30–30 form,
as shown above) is more similar to that of CS: the B3LYP/6-
311++G(d,p) calculated spatial extents for CS, non-planar TS and
planar TS are 3225, 4294 and 4327 a.u., respectively (a superposi-
tion of the calculated structures for non-planar and planar TS with
CS yielded a normalized root-mean-square-deviation ratio of
structures similarity of 1:0.92, favoring the non-planar TS form).
Hence, rearrangement of the host matrix atoms defining the pri-
mary CS occupied matrix cages is less energetically demanding
for CS ? non-planar TS than for CS ? planar TS transformation.
Secondly, because the mechanism for UV isomerization of CS into
3
.4. Non-planar vs. planar TS
As mentioned in the Introduction, the ground state structure of
TS has been a subject of continuous discussion [33,44]. It is now ac-
cepted that in gas phase the planar geometry is the global mini-
mum energy conformation [40,41], but evidence has also been
presented that non-planar conformations may easily be stabilized
by intermolecular interactions (e.g., in solutions and the pure liquid
TS [50–52]). Even for the isolated molecule, these non-planar
conformations were concluded to be stabilized in relation to the
planar form by zero-point vibrational effects [40], so that the pos-
sibility of existence of a stable non-planar conformation with
energy close to that of the planar structure and a low energy bar-
rier separating these forms appears as a real possibility. Very
unfortunately, theory has produced contradictory results [40,44]
regarding this point, with the obtained data being very much
dependent on the level of theory and basis sets used. Under these
circumstances, experimental evidence gains additional relevance.
The present investigation is a strong indication of the existence
of a second (non-planar) experimentally relevant conformer of
TS, even in gas phase. This conclusion can be extracted based on
the following observations: (a) As shown in the previous sections,
the non-planar TS form can be easily produced upon UV-photolysis
of matrix-isolated CS. According to the relative intensities of bands
in the matrices IR spectra collected immediately after irradiation
ceased, the amount of the non-planar TS form was found to
constitute about 26% of the photoproduced species. The fact that
in the matrix media this non-planar form is a minimum energy
1
TS implies mediation of the S minimum where the two phenyl
rings are nearly perpendicular to each other [1,5,6,14–20]; hence,
the ground state non-planar form can be reached firstly than the
planar structure. Finally, because of the low work temperature
(15 K), the probability of the non-planar form to survive the energy
relaxation processes following its immediate production in a vibra-
tionally excited state can be expected to be significant, and once
vibrationally relaxed it is kept stable; as observed, a temperature
increase was required to allow overcoming of the energy barrier
separating this form from the most stable TS planar form.
3.5. Natural bond interactions and electronic effects in stilbenes
Since besides the above mentioned zero-point vibrational
effects [40] electronic resonance effects can be expected to be
ünsalan, Ozan
