Molecular conformations of jet-cooled 2-methylindan and 2-phenylindan

Article abstract of DOI:10.1007/s11224-010-9630-x

The molecular conformations of jet-cooled 2-methylindan (2MI) and 2-phenylindan (2PI) have been studied using resonant-enhanced two-photon ionization spectroscopy in combination with ab initio calculations. Both axial (2MI_ax) and equatorial (2MI_eq) conformers of 2MI have been observed. A 2MI_eq/2MI_ax conformer ratio of 2.3 was estimated at 298 K, leading to the energy difference, of 0.49 kcal/mol. Ab initio calculations predicted three stable conformers of 2PI: Two equatorial conformers (2PI_eq0 and 2PI_eq90), and one axial conformer (2PI_ax). Only the axial conformer of 2PI (2PI_ax) was experimentally observed. The indan ring of 2PI_ax is slightly more planar than the indan rings of the two equatorial conformers of 2PI because of the intramolecular C_sp2-H/π interactions in 2PI_ax. The equatorial conformers of 2PI relax to the more stable axial conformer because of the high pre-expansion temperature (383 K), and relatively low barrier (1.68 kcal/mol) to axial-equatorial interconversion. The barrier (2.33 kcal/mol) to axial-equatorial interconversion in 2MI is high enough to prevent conformational relaxation at the pre-expansion temperature of 298 K. Intramolecular C-H/π interactions are found to be more important in determining the conformational preference of 2PI than 2MI; this can be attributed to the higher acidity of the C_sp2-H bond than that of C_sp3-H bond.

Full text of DOI:10.1007/s11224-010-9630-x

Struct Chem (2010) 21:939–945
DOI 10.1007/s11224-010-9630-x
ORIGINAL RESEARCH
Molecular conformations of jet-cooled 2-methylindan
and 2-phenylindan
Abdulhamid Hamza
Received: 27 October 2009 / Accepted: 17 May 2010 / Published online: 11 June 2010
Ó Springer Science+Business Media, LLC 2010
Abstract The molecular conformations of jet-cooled
2-methylindan (2MI) and 2-phenylindan (2PI) have been
studied using resonant-enhanced two-photon ionization
spectroscopy in combination with ab initio calculations.
Both axial (2MIax) and equatorial (2MIeq) conformers of
2MI have been observed. A 2MIeq/2MIax conformer ratio
of 2.3 was estimated at 298 K, leading to the energy dif-
ference, DE ¼ E2MI ꢀ E2MI , of 0.49 kcal/mol. Ab initio
Introduction
The weak non-bonded interactions of the p electron cloud
of aromatic molecules with hydrogen atoms of the C–H,
N–H, and O–H bonds have been the subject of several
experimental and theoretical investigations [1–7]. These
interactions play an important role in determining the
conformational preferences of molecules and clusters
[1–7]. The interactions are mediated by dispersion and
electrostatic forces; the latter are more important in the case
of polar N–H and O–H bonds [4]. The C–H/p interactions
are dominated by dispersion interactions. The p interactions
involving polar N–H and O–H bonds are usually stronger
than the C–H/p interactions. For example, the binding
energies of benzene–water and benzene–methane clusters
are 3.02 kcal/mol and 1.45 kcal/mol, respectively [4].
A red-shift of vibronic transitions is usually associated
with C–H/p interactions. Thus, the 610 bands of benzene–
ethylene and benzene–methane clusters are red-shifted by
64 cm-1 and 39 cm-1, respectively, relative to the 610 band
of benzene [1, 8]. Allylbenzene exists predominantly in the
eclipsed conformation, which is stabilized by the C–H/p
interactions. The electronic origin of the eclipsed con-
former of allylbenzene is red-shifted by 121 cm-1 relative
to that of the extended conformer [9]. On the other hand,
the N–H/p and O–H/p interactions are often associated
with the blue-shift of the electronic origins of the mole-
cules and clusters [5]. Thus, the electronic origin of ben-
zene–water cluster is blue-shifted with respect to that of
bare benzene by 49 cm-1 [5]. Also, the electronic origin of
benzylalcohol is blue-shifted by 50 cm-1 relative to that of
toluene [10, 11].
ax
eq
calculations predicted three stable conformers of 2PI: two
equatorial conformers (2PIeq0 and 2PIeq90), and one axial
conformer (2PIax). Only the axial conformer of 2PI (2PIax)
was experimentally observed. The indan ring of 2PIax is
slightly more planar than the indan rings of the two
equatorial conformers of 2PI because of the intramolecular
Csp2–H/p interactions in 2PIax. The equatorial conformers
of 2PI relax to the more stable axial conformer because of
the high pre-expansion temperature (383 K), and relatively
low barrier (1.68 kcal/mol) to axial–equatorial intercon-
version. The barrier (2.33 kcal/mol) to axial–equatorial
interconversion in 2MI is high enough to prevent confor-
mational relaxation at the pre-expansion temperature of
298 K. Intramolecular C–H/p interactions are found to be
more important in determining the conformational prefer-
ence of 2PI than 2MI; this can be attributed to the higher
acidity of the Csp2–H bond than that of Csp3–H bond.
Keywords Conformations ꢁ Spectroscopy ꢁ Ab initio ꢁ
2-Methylindan ꢁ 2-Phenylindan
A. Hamza (&)
Petroleum Chemistry and Engineering Program,
School of Arts and Sciences, American University of Nigeria,
PMB 2250 Yola, Nigeria
Recently, a large number of conformationally flexible
derivatives of indan have been investigated with the pri-
mary aim of determining the factors governing the
e-mail: abdulhamid.hamza@aun.edu.ng
123

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Struct Chem (2010) 21:939–945
Scheme 1 Structures of a 2PI,
b 2PI-d5, and c 2MI. The
puckering angle, p, is defined as
C1–C2–C3–C4. The torsional
angle, s, is defined as
C2’–C1’–C2–H2
conformational stabilities of organic molecules [2, 3, 6, 7,
12–14]. Only two of the six stable conformers of 1-amin-
oindan were observed in the supersonic jet experiments of
Isozaki et al. [3]. The observed conformers are stabilized
by intramolecular N–H/p interactions. The most abundant
conformers of 2-aminoindan and 2-indanol are also found
to be stabilized by the intramolecular N–H/p and O–H/p
interactions, respectively [2, 6]. The electronic origins of
the intramolecular N–H/p and O–H/p hydrogen bonded
major conformers are blue-shifted relative to the electronic
origins of the minor conformers, devoid of intramolecular
hydrogen bonding [2, 3, 6, 7, 13].
0 °C. The mixture was stirred for 2 h at room temperature
to yield 2-hyroxyl-2-methylindan. The latter was bromi-
nated with PBr3 in 20 mL of dichloromethane at 20 °C for
2 h to yield 2-bromo-2-methylindan. 2MI was synthesized
by refluxing 2-bromo-2methylindan with magnesium in
methanol. The identity of the synthesized compounds was
confirmed by GC/MS analysis.
Spectroscopy
The supersonic jet spectrometer used in this study has been
described in details elsewhere [15]. Briefly, the spectrom-
eter is composed of two differentially pumped expansion
and ionization chambers. The ionization chamber houses
the time of flight mass spectrometer; the required high
vacuum (2 9 10-7 Torr) in the ionization chamber is
achieved by using a liquid nitrogen trap 6-in. diffusion
pump. The sample source is located inside the expansion
chamber; 9-in. vapor diffusion pump is used to maintain
the pressure of about 10-3 Torr in the expansion chamber.
The sample is heated (to 298 K for 2MI and 383 K for 2PI
and 2PI-d5), and then seeded in a helium carrier gas at a
backing pressure of 1–3 atm. The free jet in the expansion
chamber is skimmed by a 1-mm skimmer, and the resulting
molecular beam in the ionization chamber is perpendicu-
larly crossed by a frequency doubled, Nd:YAG pumped
dye laser beam. Molecular ions are formed by sequential
absorption of one color resonant and ionizing photons. The
molecular ions are then detected and mass analyzed using a
time of flight mass spectrometer. Resonant-enhanced two-
photon ionization (R2PI) spectra are measured using a
boxcar integrator interfaced with a computer.
The main objective of this study was to investigate the
role of the Csp3–H/p and Csp2–H/p interactions on the
conformational stability of molecules. In order to accom-
plish this task, the following compounds were synthesized:
2-methylindan (2MI), 2-phenylindan (2PI), and 2-pheny-
lindan-d5 (2PI-d5). The structures of these compounds are
shown in Scheme 1. Supersonic jet electronic spectra of the
synthesized compounds have been recorded using reso-
nant-enhanced two-photon ionization technique, and ana-
lyzed with the support of ab initio calculations.
Experimental and computational methods
Synthesis
2-phenylindan was synthesized in three steps. Ethereal
solution of phenylmagnesiumbromide (16 mL, 1 M) was
slowly added to a solution of 2-indanone (12 mmol in
12 mL of diethylether) at 0 °C. The mixture was stirred for
2 h at room temperature to yield 11 mmol of 2-hydroxyl-2-
phenylindan. Bromination of 2-hydroxyl-2-phenylindan
(11 mmol) with 7 mmol of PBr3 in 25 mL of dichloro-
methane at 20 °C for 2 h yielded 2-bromo-2-phenylindan.
A mixture of 2-bromo-2-phenylindan (10 mmol) and
magnesium (50 mmol) in methanol (30 mL) was refluxed
for 20 h. The reaction mixture was quenched with HCl
(12 mL, 3 M). Petroleum ether was then used for the
extraction of 2PI. 2PI-d5 was synthesized following the
same procedure as for 2PI. Phenyl magnesiumbromide-d5
was made by Grignard reaction of bromobenzene-d5
and magnesium in ether. Ethereal solution of meth-
ylmagnesiumbromide (15 mmol, 1 M) was added slowly to
a solution of 2-indanone (10 mmol in 15 mL of ether) at
Ab initio calculations
Ground state optimizations and normal mode analyses of
the investigated molecules were initially performed at the
Hatree–Fock (HF) level of theory with 6-311G(d,p) basis
set. The HF/6-311G(d,p) optimized minima (stable con-
formers) and maxima (transition states) were then re-opti-
mized using the correlated Møller–Plesset perturbation
(MP2) method. It was reported that the MP2 method
describes dispersion interactions very well [9]. Since the
C–H/p interactions are mostly governed by dispersion
interactions and for the sake of brevity, only the results
of the MP2/6-311G(d,p) ground state calculations are
123

Struct Chem (2010) 21:939–945
941
2-Methylindan (2MI)
reported in this article. The MP2/6-311G(d,p) optimized
geometries were used for computing the vertical excitation
energies using the time-dependent density functional the-
ory (TDDFT) with B3LYP functional, and the configura-
tion interaction singles (CIS) methods. The CIS method
was employed for optimizing the geometries and harmonic
frequency calculations of the stable conformers of 2MI and
2PI in their first excited singlet states (S1). Gaussian03 suite
of programs [16] was used for all calculations.
The optimized structures of the minimum energy con-
formers of 2MI and the transition state between the two
minima are displayed in Scheme 2. The axial conformer
(2MIax) is predicted to be 0.24 kcal/mol more stable than
the equatorial conformer (2MIeq) at the MP2/6-311G(d,p)
level.
The resonance-enhanced two-photon ionization (R2PI)
spectrum of 2MI is displayed in Fig. 1. Two electronic
origins are identified at 36903.3 cm-1 and 36924.0 cm-1
.
The fundamental transitions of the puckering mode appear
at 73.6 cm-1 and 75.7 cm-1 from the first and second
origins, respectively. These assignments are based on the
fact that low frequency vibronic transitions due to the
puckering mode have been observed in most of the pub-
lished electronic spectra of jet-cooled indan and its deriv-
atives [2, 3, 6, 7, 12, 13]. The electronic origins at
36903.3 cm-1 and 36924.0 cm-1 are assigned to the
electronic origins of 2MIax and 2MIeq, respectively, by
comparison with the closely related allylbenzene and n-
propylbenzene [9, 10]. The electronic origins of the
eclipsed/gauche conformers (in which the substituent group
points toward the aromatic chromophores) of n-propyl-
benzene and allylbenzene are red-shifted relative to those
of the extended trans conformers.
Results and discussion
The cyclopentenyl ring of indan is not planar; therefore, a
single substitution at 2-position will lead to two conform-
ers: axial (ax) and equatorial (eq). The puckering angle, p,
and the torsional angle, s, are defined in Scheme 1. Sum-
marized in Table 1 are some of the results of the MP2/6-
311G(d,p) ground state optimizations of 2MI and 2PI. All
stable conformers of 2MI and 2PI have Cs symmetry.
Table 2 lists the computed vertical and adiabatic S0 / S1
excitation energies of all the stable conformers of 2MI and
2PI. S0 / S1 adiabatic excitation energy is the difference
in the total energies of the CIS optimized S1 minimum and
the HF optimized S0 minimum.
As reported in Table 2, the calculated vertical and adi-
abatic S1 / S0 excitation energies of 2MIax are less than
those of 2MIeq. The CIS/6-311G(d,p) computed puckering
frequency in the S1 state of 2MIax (78 cm-1) is also less
than that of 2MIeq (84 cm-1). Thus, the computed relative
S1 / S0 excitation energies and the frequencies of the
puckering mode support the assignment of the two elec-
tronic origins.
Table 1 Summary of the MP2/6-311G(d,p) calculated S0 data for the
stable conformers of 2MI and 2PI
2MI
2PI
2MIax
2MIeq
2PIax
2PIeq0
2PIeq90
Earel (kcal/mol)
0
0.24
31.8
–
0
0.67
32.1
0.0
1.95
31.6
89.1
pb (°)
sb (°)
a
32.3
–
28.6
0.0
A MIeq/MIax conformer ratio of 2.3 was estimated by
integrating the areas under the origin band of each con-
former [6]. The Boltzmann distribution law was then used
to estimate their relative energy, DE ¼ E2MI ꢀ E2MI , of
Relative to the axial conformers
b
The puckering angle, p, and the torsional angle, s, are defined in
Scheme 1
ax
eq
0.49 kcal/mol.
2-Phenylindan
Table 2 Vertical and adiabatic S1 / S0 excitation energies of the
stable conformers of 2MI and 2PI
The optimized structures of the minimum energy con-
formers of 2PI and the transition states between the minima
are displayed in Scheme 3.
Vertical S1 / S0 excitation Adiabatic S1 / S0 excitation
energies using the MP2/6-
311G(d,p) S0 structures
energies using the HF/6-
311G(d,p) S0 structures
and the CIS/6-311G(d,p)
S1 structures
Two different minima have been found along the phenyl
torsional coordinate of the potential energy surface of the
equatorial conformer of 2PI at torsional angles, s, of 0°
(2PIeq0) and 90° (2PIeq90). The MP2/6-311G(d,p) relative
energy of the 2PIeq0 and 2PIeq90 conformers is 1.28 kcal/
mol in favor of 2PIeq0. The 2PIeq90 minimum lies at only
0.06 kcal/mol below the transition state between the 2PIeq0
and 2PIeq90 (TSeq in Scheme 3).
CIS/6- TDDFT/B3LYP/
311G(d,p) 6-311G(d,p)
2MIax 48110
41848
41917
41264
41733
41846
48009
48030
47822
48015
48063
2MIeq 48150
2PIax 47787
2PIeq0 48099
2PIeq90 48145
123

942
Struct Chem (2010) 21:939–945
Scheme 2 Conformational
landscape of 2MI
Fig. 1 R2PI spectrum of 2MI
Only one minimum was found along the phenyl tor-
sional coordinate of the axial conformer of 2PI, at s = 0°.
The barrier (TSax in Scheme 3) to torsional motion is
3.84 kcal/mol. The high barrier in the axial conformer is
caused by the repulsive non-bonded interactions between
the two ortho hydrogens of the phenyl ring and the two a
hydrogens (relative to the phenyl ring) of the indan ring.
Here the closest distance between the ortho hydrogens of
the phenyl ring and the a hydrogens of the indan ring is
there are no obvious vibronic transitions that can be
attributed to more than one conformer of 2PI. The first
intense peak at 36884.4 cm-1 in the spectrum of 2PI is thus
assigned to the electronic origin of 2PI. The most likely
candidate for the origin of another conformer of 2PI is the
band at 141.6 cm-1 from the assigned origin. However, the
corresponding band in the spectrum of 2PI-d5 has a fre-
quency of 139.9 cm-1. Hence the band at 141.6 cm-1 is
not an electronic origin because of the associated isotope
effect. Isotopic substitution has very much higher effect on
the frequencies of vibronic transitions than on the relative
000 transition energies of conformers [11].
The observed 000 transition energies in R2PI spectra
correspond to transitions from the vibrationless energy
level in the S0 state to the vibrationless energy level in the
S1 and/or S2 states. Hence, they cannot be numerically
compared with the computed vertical excitation energies at
any level of theory. Computation of vertical excitation
energies neglects the effect of geometrical relaxation of
electronically excited molecules to the vibrationless energy
level of the excited electronic states. Ideally, the observed
000 transition energies should be comparable to the
˚
1.95 A at the MP2/6-311G(d,p) level of theory. It has been
reported that the HꢁꢁꢁH repulsion energy is negligible
˚
beyond 2.4 A but increases to 1 kcal/mol at HꢁꢁꢁH distance
˚
of about 2.0 A [17].
The main purpose for the synthesis of 2PI-d5 is to
facilitate and support the spectral analysis of 2PI as will be
discussed below. It should be noted that both 2PI and 2PI-
d5 have the same symmetry, but different vibrational fre-
quencies because of the higher reduced masses of the
vibrational modes of 2PI-d5. The spectra of 2PI and 2PI-d5
are displayed in Fig. 2. The frequencies of the electronic
origins and the relative frequencies of the major vibronic
bands are also shown in Fig. 2. Unlike in the case of 2MI,
123

Struct Chem (2010) 21:939–945
943
Scheme 3 Conformational
landscape of 2PI
Fig. 2 R2PI spectra of a 2PI-d5
and b 2PI
computed adiabatic excitation energies which take relaxa-
tion effects into account. TDDFT adiabatic excitation
energies cannot be computed using Gaussian03. However,
it is well known that the CIS-computed excitation energies
are overestimated by about 1 eV due to the inherent
approximations in the CIS methodology. Nevertheless, a
linear correlation between the CIS-computed adiabatic
excitation energies and the observed 000 transition energies
in a series of aromatic molecules has been found. This
permits approximate quantitative comparison of the CIS-
computed adiabatic energies and the observed 000 transition
energies in a series of aromatic molecules [19].
The computed S1 / S0 vertical and adiabatic excitation
energies of the axial conformer of 2PI (2PIax) are sub-
stantially lower than those of the equatorial conformers
(Table 2). The lower excitation energies of the 2PIax
123

944
Struct Chem (2010) 21:939–945
Fig. 3 Frontier molecular
orbitals involved in the S1 / S0
and S2 / S0 transitions of 2PIax
conformer can be attributed to the lower puckering angles,
p, of the axial conformer (Table 1). The greater planarity of
the axial conformer is most likely caused by the intramo-
lecular Csp2–H/p interactions. The values of p of the axial
and equatorial conformers of 2MI and the equatorial con-
formers of 2PI are similar. Thus, the calculations suggest
that intramolecular Csp2–H/p interactions occur only in the
2PIax conformer. The slight red-shift of the electronic ori-
gin of 2PI with respect to that of the axial conformer of
2MI (2MIax) can be attributed to the higher intramolecular
the fundamentals of puckering mode appear at 771.2 and
762.8 cm-1 in the spectra of 2PI and 2PI-d5, respectively.
Full deuteration of the phenyl ring of 2PI causes the
electronic origin of 2PI-d5 to blue-shift by only 0.4 cm-1
,
suggesting that the S1 / S0 transition of 2PI is localized on
the indan chromophore. The biggest difference between the
spectra of 2PI and 2PI-d5 is associated with the peaks at
605.4 cm-1 and 773.1 cm-1 above the electronic origins of
2PI and 2PI-d5, respectively. The two peaks are assigned to
the electronic origins of the S2 / S0 transitions of 2PI and
2PI-d5 that are localized on the phenyl and phenyl-d5
chromophores, respectively. The observed energy gap
between the S2 / S0 origins of 2PI and 2PI-d5 is
167.7 cm-1, which is close to the energy gap of
165.9 cm-1 between the electronic origins of toluene and
toluene-d5 [18]. The observation of localized S1 / S0 and
S2 / S0 transitions of 2PI indicates that the phenyl and
indan rings of 2PI are perpendicular to each other.
According to the TDDFT/B3LYP/6-311G(d,p) calcula-
tions, the S1 / S0 transition is mostly dominated by a
single transition (with CI coefficient of 0.55) from HOMO
to LUMO ? 1 orbital. As seen in Fig. 3, these two orbitals
are completely localized on the indan chromophore. The
S2 / S0 transition is dominated by two transitions:
(HOMO-1 to LUMO with CI coefficient of 0.36) and
(HOMO-1 to LUMO ? 2 with CI coefficient of 0.36).
Thus, the localized nature of the S1 / S0 and S2 / S0
transitions is supported by the TDDFT calculations.
Csp2–H/p interactions in 2PIax than intramolecular Csp3–H/
p interactions in 2MIax [1, 8, 9]. Based on these arguments,
all vibronic bands in the spectrum of 2PI are due the most
stable 2PIax conformer. Hence, only the 2PIax conformer is
populated in the supersonic expansion.
The first two CIS/6-311G(d,p) calculated lowest fre-
quency vibrational modes of the 2PIax conformer in its
optimized S1 state are associated with the puckering and
flapping modes, respectively. The calculated frequencies
(38 cm-1 and 163 cm-1) of these modes are very close to
the observed frequencies of 41.1 cm-1 and 141.6 cm-1 in
the R2PI spectrum of 2PI shown in Fig. 2b. Therefore, the
observed 41.1 cm-1 and 141.6 cm-1 fundamentals are
assigned to the puckering and flapping modes of 2PIax,
respectively. The drop in the frequencies of the puckering
and flapping modes of 2PI-d5 to 40.2 cm-1 and
139.9 cm-1, respectively, is caused by the higher reduced
masses of the modes in 2PI-d5 than in 2PI. The relatively
intense vibronic transition at *700 cm-1 in the spectra of
indan derivatives is associated with the in-plane bending of
indan ring [2, 3]. Similar transitions also appear in the
Conformational relaxation of 2PI and 2MI
spectra of 2PI and 2PI-d5 at 730.1 cm-1 and 723.0 cm-1
,
Very fast relaxation of the 2PIeq90 conformer to the more
stable 2PIeq0 conformer is anticipated due to the very low
respectively. The combinations of these fundamentals with
123

Struct Chem (2010) 21:939–945
945
Acknowledgments The author is grateful to Dr. John R. Cable,
Bowling Green State University, Ohio, for providing the facility used
for this research.
barrier of 0.06 kcal/mol to such transition (TSeq in
Scheme 3). Therefore, only two conformers (2PIeq0 and
2PIax) of 2PI are expected to be observed in the supersonic
jet experiments. However, the most intriguing aspect of
this study appears to be the absence of 2PIeq0 in the
supersonic jet expansion. It has been observed that barriers
to conformational interconversions in a supersonic jet can
be surmounted if the barriers are less than 2kT [14]. Where
k is the Boltzmann constant; T is the pre-expansion tem-
perature (383 K for 2PI and 298 K for 2MI). The computed
barrier to axial–equatorial interconversion in 2PI (TSax-eq
in Scheme 3) is 1.68 kcal/mol above the 2PIeq0 local
minimum. This barrier is about the same as 2kT (1.52 kcal/
mol) of 2PI. Meanwhile, the barrier to axial–equatorial
interconversion in 2MI (TSax-eq in Scheme 2) is 2.33 kcal/
mol above the 2MIeq minimum. Thus, the barrier in 2MI is
substantially larger than 2kT (1.16 kcal/mol) of 2MI. The
lower barrier in 2PI can be attributed to the higher stabi-
lizing effect of the Csp2–H/p interactions in the transition
state of 2PI (TSax-eq in Scheme 3) than in the transition
state of 2MI (TSax-eq in Scheme 2). Therefore, conforma-
tional relaxation of the equatorial isomers of 2PI occurs
because of the high pre-expansion temperatures and rela-
tively low barrier to axial–equatorial interconversion.
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17. Wiberg KB, Castejon H, Bailey WF, Ochterski J (2000) J Org
Chem 65:1181
Both axial (2MIax) and equatorial (2MIeq) conformers of
2MI have been observed. A 2MIeq/2MIax conformer ratio
of 2.3 was estimated at 298 K, leading to the energy dif-
ference, DE ¼ E2MI ꢀ E2MI , of 0.49 kcal/mol. Only the
ax
eq
axial conformer of 2PI was observed in the supersonic jet
expansion. The equatorial conformers of 2PI relax to the
more stable axial conformer because of the high pre-
expansion temperatures and relatively low barrier to axial–
equatorial interconversion. The barrier to axial–equatorial
interconversion in 2MI is high enough to prevent confor-
mational relaxation at the pre-expansion temperature of
298 K. The results of this study show that intramolecular
C–H/p interactions are more important in determining the
conformational stability of 2PI than 2MI. This can be
attributed to the higher acidity of the Csp2–H bond
(pKa * 44) than that of Csp3–H bond (pKa * 50). The
S1 / S0 and S2 / S0 transitions of 2PI are found to be
completely localized on the indan and phenyl chromoph-
ores, respectively, suggesting that the phenyl and indan
rings of 2PI are perpendicular to each other.
18. Walter JB, Ram SR (1994) Can J Phys 72:1225
19. Sotoyama W, Sato H, Matsuura A, Sawatari N (2006) J Mol
Struct 759:165
123