Conformation of K+(Crown Ether) Complexes Revealed by Ion Mobility-Mass Spectrometry and Ultraviolet Spectroscopy

Article abstract of DOI:10.1021/acs.jpca.0c09068

The conformation and electronic structure of dibenzo-24-crown-8 (DB24C8) complexes with K+ ion were examined by ion mobility-mass spectrometry (IM-MS), ultraviolet (UV) photodissociation (UVPD) spectroscopy in the gas phase, and fluorescence spectroscopy in solution. Three structural isomers of DB24C8 (SymDB24C8, Asym1DB24C8, and Asym2DB24C8) in which the relative positions of the two benzene rings were different from each other were investigated. The IM-MS results at 86 K revealed a clear separation of two sets of conformers for the K+(SymDB24C8) and K+(Asym1DB24C8) complexes whereas the K+(Asym2DB24C8) complex revealed only one set. The two sets of conformers were attributed to the open and closed forms in which the benzene-benzene distances in the complexes were long (>6 ?) and short (<6 ?), respectively. IM-MS at 300 K could not separate the two conformer sets of the K+(SymDB24C8) complex because the interconversion between the open and closed conformations occurred at 300 K and not at 86 K. The crown cavity of DB24C8 was wrapped around the K+ ion in the complex, although the IM-MS results availed direct evidence of rapid cavity deformation and the reconstruction of stable conformers at 300 K. The UVPD spectra of the K+(SymDB24C8) and K+(Asym1DB24C8) complexes at ～10 K displayed broad features that were accompanied by a few sharp vibronic bands, which were attributable to the coexistence of multiple conformers. The fluorescence spectra obtained in a methanol solution suggested that the intramolecular excimer was formed only in K+(SymDB24C8) among the three complexes because only SymDB24C8 could possibly assume a parallel configuration between the two benzene rings upon K+ encapsulation. The encapsulation methods for K+ ion (the wraparound arrangement) are similar in the three structural isomers of DB24C8, although the difference in the relative positions of the two benzene rings affected the overall cross-section. This study demonstrated that temperature-controlled IM-MS coupled with the introduction of appropriate bulky groups, such as aromatic rings to host molecules, could reveal the dynamic aspects of encapsulation in host-guest systems.

Full text of DOI:10.1021/acs.jpca.0c09068

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Article
Conformation of K+(Crown Ether) Complexes Revealed by Ion
Mobility−Mass Spectrometry and Ultraviolet Spectroscopy
Sota Tainaka, Tomoyuki Ujihira, Mayuko Kubo, Motoki Kida, Daisuke Shimoyama, Satoru Muramatsu,
Manabu Abe, Takeharu Haino, Takayuki Ebata, Fuminori Misaizu, Keijiro Ohshimo,*
and Yoshiya Inokuchi*
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ABSTRACT: The conformation and electronic structure of dibenzo-24-crown-8 (DB24C8) complexes with K+ ion were examined
by ion mobility−mass spectrometry (IM−MS), ultraviolet (UV) photodissociation (UVPD) spectroscopy in the gas phase, and
fluorescence spectroscopy in solution. Three structural isomers of DB24C8 (SymDB24C8, Asym1DB24C8, and Asym2DB24C8) in
which the relative positions of the two benzene rings were different from each other were investigated. The IM−MS results at 86 K
revealed a clear separation of two sets of conformers for the K+(SymDB24C8) and K+(Asym1DB24C8) complexes whereas the
K+(Asym2DB24C8) complex revealed only one set. The two sets of conformers were attributed to the open and closed forms in
which the benzene−benzene distances in the complexes were long (>6 Å) and short (<6 Å), respectively. IM−MS at 300 K could
not separate the two conformer sets of the K+(SymDB24C8) complex because the interconversion between the open and closed
conformations occurred at 300 K and not at 86 K. The crown cavity of DB24C8 was wrapped around the K+ ion in the complex,
although the IM−MS results availed direct evidence of rapid cavity deformation and the reconstruction of stable conformers at 300
K. The UVPD spectra of the K+(SymDB24C8) and K+(Asym1DB24C8) complexes at ∼10 K displayed broad features that were
accompanied by a few sharp vibronic bands, which were attributable to the coexistence of multiple conformers. The fluorescence
spectra obtained in a methanol solution suggested that the intramolecular excimer was formed only in K+(SymDB24C8) among the
three complexes because only SymDB24C8 could possibly assume a parallel configuration between the two benzene rings upon K+
encapsulation. The encapsulation methods for K+ ion (the “wraparound” arrangement) are similar in the three structural isomers of
DB24C8, although the difference in the relative positions of the two benzene rings affected the overall cross-section. This study
demonstrated that temperature-controlled IM−MS coupled with the introduction of appropriate bulky groups, such as aromatic
rings to host molecules, could reveal the dynamic aspects of encapsulation in host−guest systems.
challenging to apply XRD to all of the complexes because it
requires the crystallization of the complexes. Another method
of determining the structure of crown ether complexes is ion
mobility−mass spectrometry (IM−MS) in the gas phase.7−24
Most previous IM−MS studies were focused on elucidating the
INTRODUCTION
■
Crown ethers are widely employed as host molecules or
building blocks of supramolecular systems. One of the
characteristic functions of crown ethers is ion selectivity.1,2
The most famous combination of crown ethers with metal ion
guests is that between 18-crown-6 (18C6) and K+. It has been
suggested that an optimum size matching of 18C6 with a K+
ion could result in K+ ion selectivity of 18C6 among the alkali
metal ions and that the conformation of the crown part could
be key to effective encapsulation. The encapsulation structures
of crown ether complexes have been mainly investigated in
crystal forms by X−ray diffraction (XRD).3−6 However, it is
Received: October 6, 2020
Revised: November 9, 2020
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conformations of biomolecular ions. Regarding IM−MS of
host−guest or supramolecular systems, Bowers and coworkers
reported experiments on 18C6 complexes with alkali metal
ions.25 The arrival time distribution (ATD) of the K+(18C6)
complex revealed one component, which indicated the
presence of a single structure or a set of very similar
structures.25 The capacity of IM−MS to distinguish different
conformations for host−guest systems has not been demon-
strated. Dearden and coworkers also reported the MS−MS
studies on host−guest complexes.26,27 More recently, crown
ether complexes with metal ions were investigated in the gas
phase through spectroscopic techniques.28−39 Ultraviolet
photodissociation (UVPD) spectroscopy performed under
cold, gas-phase conditions could highly distinguish several
conformers.28,29,37−42 However, the UVPD spectra of larger
systems tend to reveal broad spectral features, which limited
the distinction of different conformers or the determination of
their structures.43 Rizzo and coworkers reported pioneering
works on the cold-ion spectroscopy of biomolecular ions
selected by IM.44,45 They elaborated on the relation between
the conformation and spectroscopic features.
IM−MS to separately detect the conformers and (2) the effect
of the difference in the relative positions of the benzene rings
on the electronic and geometric structures of the K+(DB24C8)
complexes.
EXPERIMENTAL AND COMPUTATIONAL
METHODS
■
UVPD Spectroscopy in the Gas Phase and Fluo-
rescence Spectroscopy in Solution. The experimental
details of UVPD spectroscopy, which was performed at
Hiroshima University, were presented in our previous
p a p e r s . 3 9 , 4 1 B r i e fl y , t h e K + ( S y m D B 2 4 C 8 ) ,
K+(Asym1DB24C8), and K+(Asym2DB24C8) complexes
were produced from an electrospray ionization (ESI) ion
source utilizing methanol solutions of KCl and SymDB24C8,
Asym1DB24C8, or Asym2DB24C8 (∼100 μM each). The
complexes were introduced into a cold, Paul-type quadrupole
ion trap (QIT). QIT was cooled to ∼4 K by a He cryostat with
a continuous flow of He buffer gas. The ions were stored in
QIT for ∼90 ms to be translationally and internally cooled by
collision with the cold He buffer gas. The vibrational
temperature of the trapped ions was estimated to be ∼10
K.39 The ions other than the parent ones of interest were
removed from QIT by applying a radio frequency (RF)
potential to the entrance end-cap.46 The cold complexes were
irradiated afterward by a UV radiation. The resulting fragment
ions were mass-analyzed and detected with a home-made time-
of-flight (TOF) mass spectrometer.47 The UVPD spectra of
the K+ (SymDB24C8), K+ (Asym1DB24C8), and
K+(Asym2DB24C8) complexes were obtained by plotting
the yields of the fragment K+ ions against the wavenumber of
the UV laser. The UV laser was obtained by the second-
harmonic generation employing the fundamental output of a
dye laser (Continuum ND6000) pumped with the third
harmonic of an Nd3+:YAG laser (Continuum Surelite II). The
fluorescence spectra of SymDB24C8, Asym1DB24C8, and
Asym2DB24C8 dissolved in methanol with/without KCl salt
were observed at ∼300 K utilizing a commercial fluorescence
spectrometer (HORIBA Fluoromax-4).
In this study, we investigated the conformation of K+
complexes with three structural isomers of dibenzo-24-
crown-8 (DB24C8) by IM−MS and UVPD spectroscopy in
the gas phase. Figure 1 shows the structural isomers
Figure 1. Structural isomers of DB24C8 (SymDB24C8,
Asym1DB24C8, and Asym2DB24C8).
(SymDB24C8, Asym1DB24C8, and Asym2DB24C8) of the
studied DB24C8. In these isomers, the relative positions of the
benzene rings were different. We have previously reported that
the K+(SymDB24C8) complex exhibited broad UV absorption
even under cold, gas-phase conditions, and this was
attributable to the formation of an intramolecular excimer
upon irradiation by UV light.43 Conversely, the
K+(SymDB24C8) complex has been suspected to exhibit
another type of conformation because weak, sharp bands also
appeared from the broad absorption.43 However, these sharp
bands were too weak to be identified as different conformers
through double-resonance spectroscopies.28,29,39−42 The IM−
MS experiment is free from the restriction originating from the
optical transition such as the oscillator strength, and it can
separate different conformers and determine the abundance
ratio quantitatively. Here, we examined (1) the capacity of
Ion Mobility−Mass Spectrometry (IM−MS). The
present IM experiments were performed with a home-made
IM mass spectrometer at Tohoku University (Figure 2).24 The
apparatus consisted of four chambers that were separately
pumped to vacuum. The following components were placed in
each chamber: (i) an ESI ion source, (ii) QIT and an ion−drift
cell with an ion funnel, (iii) an octopole ion guide and
acceleration electrodes for TOF−MS, and (iv) reflection
electrodes and a microchannel plate (MCP) for ion detection.
Each of the three DB24C8 structural isomers and KCl salt
were dissolved in methanol to prepare solutions with
Figure 2. Schematic of the IM mass spectrometer at Tohoku University.
B
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concentrations of ∼100 μM each. The solution was loaded into
a metal capillary by a syringe pump at 1 μL/min, after which
high voltage (+3.3 kV) was applied to the capillary. The exit of
the capillary was positioned ∼5 mm from the entrance of a
heated desolvation capillary that was maintained at ∼320 K.
The exit of the desolvation capillary was inserted into the first
vacuum chamber whose pressure was 0.25 Torr. The first
chamber was connected to the second chamber via a conical
skimmer.
Computational Methods. Further, we conducted quan-
tum chemical calculations on the K+(SymDB24C8),
K+(Asym1DB24C8), and K+(Asym2DB24C8) complexes.
The initial conformational search was performed by the
CONFLEX High-Performance Conformation Analysis pro-
gram with an MMFF94s force field.51−54 The structures
obtained by the initial search were further optimized by the
Gaussian 09 program package at the M05-2X/6-311++G(d,p)
level of theory.55 The vibrational analysis was also performed
at the same calculation level.
After passing through the skimmer, the ions were introduced
into QIT (Jordan TOF Products, C-1251) in the second
chamber. The ions accumulated for 90 ms in the ion trap.
Afterward, the ions were injected into the ion drift cell by a
pulsed electric field with a kinetic energy of 10 eV in the
laboratory frame at a given time (t = t0). The repetition rate of
ion injection was 10 Hz. The total drift length from the
entrance to exit apertures of the cell was 327 mm. The
diameters of the two apertures were 2.0 mm each. In the cell, a
static electric field, E = 4.6 V/cm, was applied for ion drifting.
To collect the ions, the ion funnel was attached to the end of
the cell.48 Additionally, a cryogenic jacket surrounding the
ion−drift cell was filled with liquid nitrogen. A similar
cryogenic jacket design was reported by May and Russell.49
In these experiments, the temperature of the He buffer gas was
86 K, and its pressure was maintained at 0.49 Torr through a
variable leak valve. Therefore, the E/N value was 8.3 Td (N is
the number density of the buffer gas, 1 Td = 10−17 V cm2).
The pressures in the second and third chambers were 1 × 10−4
and 6 × 10−5 Torr, respectively.
After passing through the ion−drift cell, the ions were
transported through the octopole ion guide to the acceleration
region of the TOF mass spectrometer. The ions were
reaccelerated at a kinetic energy of ∼1.4 keV into the
reflectron TOF mass spectrometer by applying the pulsed
electric field to the acceleration electrodes at a given time later
(Δt) from the pulse for the injection into the drift cell (t0),
namely t = t0 + Δt. This delay time between the two pulses, Δt,
is defined as the “arrival time”, and it was controlled by a
digital delay/pulse generator. This arrival time consisted of the
drift times in three regions: (i) the region between the center
of QIT and the entrance of the cell, (ii) the one inside the cell,
and (iii) that between the exit of the cell and the acceleration
region of the TOF mass spectrometer. To evaluate the drift
velocity of the cell to satisfy the measured arrival time, we
calculated the drift time in the three regions by solving the
equations of the motion of the ions. Therefore, we obtained
the time spent by an ion in the cell from the measured arrival
time. The drift time in the cell depends on the collision cross
sections (CCSs) between the ions and He atoms. Ions with
different CCSs arrived at the acceleration region of the
spectrometer at different times. Finally, the ions were mass-
analyzed by a reflectron TOF mass spectrometer. The pressure
in the chamber of the TOF spectrometer was 1 × 10−6 Torr.
During IM−MS measurement, we obtained a series of TOF
mass spectra sequentially by scanning the arrival times.
Resultantly, the ions with different CCSs were separately
detected at different arrival times in a two-dimensional plot of
TOF versus arrival time. We obtained ATD by plotting the ion
intensity of a given TOF peak against a function of the arrival
time. The mobility resolution of the apparatus, which was
estimated from ATD of Li+ ion-encapsulated fullerene (Li+@
C60) with a single isomer, was ∼30.50
The collision integrals of the complex ions with a He atom
were calculated by the trajectory method that is included in the
MOBCAL program.56 When an ion and a He atom were
considered as hard spheres without internal states, the collision
integral was reduced to the hard-sphere CCS. In the trajectory
method, the interaction potential between the ion and a He
atom was required. This interaction potential comprised the
sum of the interaction potentials between the constituent
atoms of the complexes (H, C, O, K atoms) and a He atom,
ViX−He(ri), as shown in eq 1:
Ä
Å
Å
Å
Å
Å
Å
Å
Å
Å
Å
É
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
12
6
X−He
X−He
i
j
j
j
j
j
y
i
j
y
z
z
z
z
z
σ
σ
z
z
z
z
z
VX−He(r) = 4εX−He
− j
j
j
i
i
j
r
r
i
i
k
{
k
{
Å
Ñ
Å
Ç
Ñ
Ö
qi2
αe2
−
(X = H, C, O, K)
r4
2
(1)
i
where ri is the distance between the ith constituent atom and a
He atom, εX−He and σX−He are the Lennard−Jones parameters,
α is the polarizability of a He atom, and qi is the partial atomic
charge on the constituent atoms. On the right side of the
equation, the first term represents the Lennard−Jones
potential between neutrals and the second represents the
charge−induced dipole interaction between qi and a He atom.
Here, we adopted the values determined in the previous study
for εX−He and σX−He (X = H, C, O).57 For εK−He, we adopted
the interaction energy of the K−He complex of the optimized
structure. Additionally, we adopted the bond distance in the
K+−He complex for σK−He assuming that the partial atomic
charge of the K atom is +1. The CCSD(T)/6-311+G(d,p)
level was employed for the calculations of K−He and K+−He.
The values of εX−He and σX−He adopted here are summarized in
Table 1. The partial atomic charges, qi, were calculated by
natural population analysis. Furthermore, 86 K was adopted as
the temperature value in the trajectory calculations.
Table 1. Values of εX−He and σX−He Adopted in This Study
X
ε
X−He/meV
σ
X−He/Å
H
C
O
K
0.6175
1.3266
1.0720
0.5151
2.2610
3.0126
2.4344
2.6368
Chemicals. Commercially available SymDB24C8 was
purchased and utilized without further purification.
Asym1DB24C858 and Asym2DB24C858 were synthesized by
previously reported modified procedures. All of the reagents
and solvents were of commercial reagent grade and were
utilized without further purification except otherwise noted.
Dry DMF and DMSO were obtained by distillation over CaH2.
1
The H and 13C NMR spectra were recorded on a Varian
mercury-300 spectrometer (operating at 300 and 75 MHz for
C
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1H and 13C NMRs, respectively) employing CDCl3 as the
solvent. All of the spectra were recorded at 298 K, and the
chemical shifts were reported on the delta scale in ppm relative
RESULTS AND DISCUSSION
UVPD Spectra in the Gas Phase and Fluorescence
Spectra in Solution. Figure 3 shows the UVPD spectra of
■
1
to CHCl3 (δ = 7.26 for H and 77.2 for 13C). The infrared
(IR) spectra were recorded on a HORIBA FT-720
spectrometer. Preparative separations were performed by silica
gel gravity column chromatography (silica gel 60 N (spherical,
neutral)). Recycling preparative gel permeation chromatog-
raphy (GPC)−high-performance liquid chromatography
(HPLC) (GPC−HPLC) separations were performed on JAI
LC-908s employing preparative JAIGEL-2H, 2H, and 1H
columns in series. High-resolution MS (HRMS) was
performed on a Thermo Fisher Scientific LTQ Orbitrap XL
spectrophotometer, and the spectra were obtained by ESI
method.
Synthesis of 6,7,9,10,12,13,15,16,23,24,26,27-
Dodecahydrodibenzo[B,k][1,4,7,10,13,16,19,22]-
octaoxacyclotetracosine (a). A mixture of bis[2-(o-
hydroxyphenoxy)ethyl]ether (0.950 g, 3.27 mmol) and
K2CO3 (1.06 g, 7.67 mmol) in DMSO (8.95 mL) was stirred
with heating at 363 K. To the mixture was added bis(p-
toluenesulfonyl) tetraethylene glycol (0.895 g, 1.78 mmol) in
DMF (8.95 mL) within 3 h. After heating for 5 d, the reaction
mixture was acidified to pH 5−6 with HCl and extracted with
CH2Cl2. The organic layer was dried over Na2SO4, and the
solvent was removed in vacuo. Τhe crude product was purified
by column chromatography with alumina as the adsorbent
(hexane/ethyl acetate) to yield Asym1DB24C8 as a colorless
oil (0.490 g, 33%).
1H NMR (300 MHz, CDCl3): δ 6.91 (m, 8H), 4.20 (t, 4H, J
= 5.0 Hz), 4.15 (t, 4H, J = 3.7 Hz), 4.00 (t, 4H, J = 5.0 Hz),
3.88 (t, 4H, J = 3.7 Hz), 3.78 (m, 4H), and 3.70 (m, 4H) ppm;
13C NMR (75 MHz, CDCl3): δ 149.3, 148.9, 121.9, 121.5,
115.6, 114.2, 71.1, 70.7, 70.3, 69.8, 69.6, and 69.1 ppm;
Fourier-transform IR (FT−IR)−attenuated total reflection
(ATR) (neat): ν 2929, 1594, 1506, 1254, and 930 cm−1;
HRMS (ESI-Orbitrap): calcd for C24H32O8Na m/z 471.1989
[M + Na]+, found m/z 471.1982.
Synthesis of 6,7,9,10,12,13,15,16,18,19,26,27-
Dodecahydrodibenzo[B,h][1,4,7,10,13,16,19,22]-
octaoxacyclotetracosine (b). A mixture of 1,2-bis(o-
hydroxyphenoxy)ethane (0.0925 g, 0.375 mmol) and K2CO3
(0.103 g, 0.745 mmol) in DMF (1.84 mL) was stirred with
heating at 363 K. To the mixture was added bis(p-
toluenesulfonyl) pentaethylene glycol (0.207 g, 0.377 mmol)
in DMF (1.84 mL) within 3 h, and the mixture was heated for
3 d. The mixture was acidified to pH 5−6 with HCl and
extracted with CH2Cl2. The organic layer was dried over
Na2SO4, and the solvent was removed in vacuo. Τhe crude
product was purified by a recycling preparative GPC−HPLC
system to yield Asym2DB24C8 as a colorless oil (0.0500 g,
30%).
Figure 3. UVPD spectra of (a) K+(SymDB24C8), (b)
K+(Asym1DB24C8), and (c) K+(Asym2DB24C8). The spectrum of
the K+(SymDB24C8) complex was adopted from the previous study
(ref 43, Copyright 2018 John Wiley and Sons). Sharp bands were also
observed on the broad components (black arrows in panels a and b).
the DB24C8 complexes with K+ ions observed under cold
(∼10 K) gas-phase conditions. The UVPD spectrum of the
K+(SymDB24C8) complex (Figure 3a), which was reproduced
from our previous study,43 was employed for comparison with
those of the K+(Asym1DB24C8) and K+(Asym2DB24C8)
complexes (Figure 3b,c). The K+(SymDB24C8) complex
exhibited a dominant conformer that displayed broad spectral
features, which related to the formation of the intramolecular
excimer.43 Weak, sharp vibronic bands in addition to the broad
features were also observed around 36 150 cm−1 (black arrow
in Figure 3a). These bands were assigned to a minor
conformer, indicating the coexistence of multiple con-
formers.43 The UVPD spectrum of K+(Asym1DB24C8)
(Figure 3b) exhibited broad absorption in the 36 000−
37 400 cm−1 region. Additionally, a few sharp bands were
also detected on the broad component of the
K+(Asym1DB24C8) complex (one of the sharp bands is
represented by an arrow at 36 603 cm−1 in Figure 3b). These
spectral features also implied the coexistence of multiple
conformers of K+(Asym1DB24C8). Regarding the
K+(Asym2DB24C8) complex, the UVPD spectrum exhibited
absorption in the 36 000−37 400 cm−1 region. Resolved
vibronic structures could be observed around 36 500 cm−1,
1H NMR (300 MHz, CDCl3): δ 6.94 (m, 8H), 4.38 (s, 4H),
4.18 (t, 4H, J = 4.7 Hz), 3.85 (t, 4H, J = 4.7 Hz), 3.72 (m,
4H), and 3.62 (m, 8H) ppm; 13C NMR (75 MHz, CDCl3): δ
149.5, 149.1, 122.2, 121.8, 116.2, 115.3, 71.1, 70.9, 70.9, 69.9,
69.4, and 68.7 ppm; FT−IR−ATR (neat): ν 2870, 1592, 1497,
1253, 1113, and 930 cm−1; HRMS (ESI-Orbitrap): calcd for
C24H32O8Na m/z 471.1989 [M + Na]+, found m/z 471.1988.
D
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although the spectrum was highly congested. It is difficult to
apply double-resonance spectroscopic techniques to these
complexes and determine the structure and number of their
conformers, because the UV absorption of multiple conformers
seems to be overlapped to each other.41,42
Figure 4 shows the fluorescence spectra of SymDB24C8,
Asym1DB24C8, and Asym2DB24C8 that were dissolved in
K+(Asym2DB24C8) complex showed Stokes-shifted excimer
emissions that were observed in the K+(SymDB24C8)
complex. These fluorescence results indicated that the
K+(Asym1DB24C8) or K+(Asym2DB24C8) complex did not
form an intramolecular excimer in solution. Regarding
SymDB24C8, the two benzene rings of the SymDB24C8
component drew close to each other upon the formation of a
complex with the K+ ion.43 For Asym1DB24C8 and
Asym2DB24C8, the relative positions of the benzene rings
on the 24C8 component were unsuitable for the formation of
the intramolecular excimer.
IM−MS Results. Figure 5 shows the ATDs of
K + ( S y m D B 2 4 C 8 ) , K + ( A s y m 1 D B 2 4 C 8 ) , a n d
K+(Asym2DB24C8) observed at 86 K. Each ATD was fitted
by Gaussian functions with a full width at half maximum
(FWHM) of 90 μs. This width was determined from the
mobility resolution of the apparatus, as aforementioned.
Regarding K+(SymDB24C8) and K+(Asym1DB24C8) (Figure
5a,b), the ATD curves can be reproduced very well by two
Gaussian functions, indicating the coexistence of two con-
formers or two sets of very similar conformers. The interval of
the two Gaussian components was shorter for
K+(Asym1DB24C8) than for K+(SymDB24C8). The ATD
results in Figure 5a,b indicated that the interconversion
between the two conformers did not occur in
K+(SymDB24C8) and K+(Asym1DB24C8) at 86 K. In
contrast, ATD of K+(Asym2DB24C8) could be fitted by one
Gaussian function. The experimental CCSs of the complexes
were determined from the positions of the Gaussian
components in the ATD data. The CCS values of the two
group conformations were 165.3 (S-a) and 174.4 Å2 (S-b) for
K+(SymDB24C8) and 167.0 (AS1-a) and 173.5 Å2 (AS1-b) for
K+(Asym1DB24C8). The K+(Asym2DB24C8) complex ex-
hibited maximum distribution at 173.1 Å2 (AS2-a). The CCS
values are listed in Table 2.
Geometric and Electronic Structures of the
K+(DB24C8) Complexes. We determined the conformation
of the complexes by comparing the experimental and
calculated CCS values. Figure 6 shows some representatives
of the stable structures of K+(DB24C8) obtained from the
geometry optimization at the M05-2X/6-311++G(d,p) level.
Further, we compared the stability of the stable conformers
based on the relative Gibbs energy at 86 K (the same
temperature as that of the IM−MS experiments). The
conformations of the complexes were characterized by the
distance between their benzene rings. Regarding the
K+(SymDB24C8) complex, the most stable structure (isomer
sym-a (Figure 6a)) exhibited benzene−benzene distance of
3.82 Å. The calculated CCS of this conformer was 163.5 0.6
Å2. The second most stable structure (isomer sym-b (Figure
6b)) exhibited a longer benzene−benzene distance (9.33 Å)
than sym-a. This resulted in a larger CCS value (172.8 0.8
Å2) than that of sym-a. We named these isomers (sym-a and
sym-b) closed and open conformers, respectively. The
geometry optimization of K+(Asym1DB24C8) also afforded
closed and open conformers. The most stable structure of
K+(Asym1DB24C8), isomer asym1-a (Figure 6c), exhibited a
closed conformation with benzene−benzene distance of 4.72
Å. The second and third most stable structures of
K+(Asym1DB24C8), isomers asym1-b and asym1-c (not
shown), also exhibited closed conformation. The calculated
theoretical CCS of these three conformers was ∼166 Å2. The
fourth most stable structure, isomer asym1-d (Figure 6d)
Figure 4. Fluorescence spectra of (a) SymDB24C8, (b)
Asym1DB24C8, and (c) Asym2DB24C8 dissolved in methanol.
The spectra of SymDB24C8 were adopted from the previous study
(ref 43, Copyright 2018 John Wiley and Sons). The excitation
wavelength was 275 nm.
methanol with/without KCl salt. The fluorescence spectra of
SymDB24C8 (Figure 4a) were reproduced from the previous
report for comparison.43 The fluorescence spectrum of
SymDB24C8 with KCl (red curve in Figure 4a) displayed a
Stokes-shifted fluorescence peak at ∼350 nm, and this was
assigned to the intramolecular excimer of the
K+(SymDB24C8) complex formed upon UV irradiation.43
The fluorescence spectra of Asym1DB24C8 and
Asym2DB24C8 are displayed in Figure 4b,c. The difference
spectra between the fluorescence spectra with/without KCl salt
(blue curves in Figure 4b,c) exhibited maxima at ∼287 and
∼292 nm for Asym1DB24C8 and Asym2DB24C8, respec-
tively, whereas the fluorescence spectra without KCl (black
curves in Figure 4b,c) displayed a maximum at ∼300 nm for
both Asym1DB24C8 and Asym2DB24C8. These results
indicated that the S1−S0 transitions of Asym1DB24C8 and
Asym2DB24C8 slightly shifted to higher frequencies upon the
formation of complexes with the K+ ion. The blue-shift of the
S1−S0 transition for the potassiated species was also observed
for SymDB24C8.43,59 Neither the K+(Asym1DB24C8) nor
E
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Table 2. Experimental CCS Values (Å2) Determined by the
a
IM−MS Experiments with He Gas at 86 K (DTCCSHe, 86
)
experimental
theoretical
DTCCSHe,86
CCS (Å2)
Bz−Bz
complexes
K+(SymDB24C8)
(Å2)
(conformers)
distance (Å)
3.82
165.3 0.4
(S-a)
163.5 0.6
(sym-a)
174.4 0.5
(S-b)
172.8 0.8
9.33
4.72
9.05
6.62
(sym-b)
K+(Asym1DB24C8) 167.0 0.4
(AS1-a)
165.9 0.8
(asym1-a)
171.3 0.6
(asym1-d)
171.1 0.8
(asym2-a)
173.5 0.6
(AS1-b)
K+(Asym2DB24C8) 173.1 0.3
(AS2-a)
a
Theoretical CCS values (Å2) of the optimized structures calculated
employing the MOBCAL program. Distance between the two
benzene rings in the complexes (Å).
Figure 6. (a and b) Stable conformers of K+(SymDB24C8). (c and d)
Stable conformers of K+(Asym1DB24C8). (e) Most stable conformer
of K+(Asym2DB24C8). Calculations were performed at the M05-2X/
6-311++G(d,p) level. The numbers in black and red show the
benzene−benzene distance and average distance between the K+ ion
and O atoms, respectively.
Figure 5. ATDs of (a) K+(SymDB24C8), (b) K+(Asym1DB24C8),
and (c) K+(Asym2DB24C8) (open circles). The ATD curves were
reproduced employing Gaussian functions (red curves). The sum of
the components is drawn by black curves. The pressure and
temperature of the He buffer gas were (a) 0.491 0.004 Torr and
85.95 0.30 K, (b) 0.484 0.004 Torr and 85.79 0.67 K, and (c)
0.490 0.001 Torr and 85.77 0.07 K.
benzene distance and CCS were 6.62 Å and 171.1 Å2,
respectively. These values were rather close to those of the
open conformations of K+ (SymDB24C8) and
K+(Asym1DB24C8). Table 2 lists the experimental and
theoretical values of CCS and the benzene−benzene distances
of K+ (SymDB24C8), K+ (Asym1DB24C8), and
K+(Asym2DB24C8).
In the quantum chemical calculations of the complexes, we
obtained many stable conformers other than those in Figure 6.
Figure 7 displays the relative Gibbs energy of the stable
could be classified as an open conformer (the benzene−
benzene distance was 9.05 Å). The theoretical CCS of isomer
asym1-d was 171.3 Å2, which was larger than those of the
closed ones (∼166 Å2). Regarding K+(Asym2DB24C8), the
benzene−benzene distances of the stable conformers did not
vary greatly. The most stable structure of K+(Asym2DB24C8)
is shown in Figure 6e (isomer asym2-a). The benzene−
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As seen in Figures 5 and 7, CCS of K+(DB24C8) strongly
depended on the structural isomers or relative positions of the
benzene rings. Previous reports suggested that larger crown
ethers, such as 24-crown-8 and 30-crown-10, wrap around the
metal ions to effectively hold the metal ions.4 Regarding
K+(SymDB24C8), this “wraparound” arrangement coinciden-
tally resulted in the effective interaction between the benzene
rings, thereby facilitating the formation of an intramolecular
excimer upon UV excitation.43 Hence, the positions of the
benzene rings in DB24C8 affected both the sizes (the CCS
values) of the complexes and their electronic structures. Benzo-
crown ethers assume planar forms of the O−C−C−O bond at
the benzene rings,29,59,61 which strongly restricted the
conformation of their crown rings. Regardless, the holding
manner of K+ ion by the 24C8 cavity was quite similar among
K + ( S y m D B 2 4 C 8 ) , K + ( A s y m 1 D B 2 4 C 8 ) , a n d
K+(Asym2DB24C8), as shown in Figure 6. The crown ring
of the 24C8 part wrapped the K+ ion irrespective of the
positions of the benzene rings in the 24C8 cavity. The
estimated average distance between K+ ion and the O atoms in
the stable conformers was ∼2.80 Å for all of the stable
conformers of K+(SymDB24C8), K+(Asym1DB24C8), and
K+(Asym2DB24C8). This also implied that the encapsulation
processes of all of the K+(DB24C8) complexes were similar.
Hence, the difference in CCSs of K+(SymDB24C8),
K+(Asym1DB24C8), and K+(Asym2DB24C8) was not due
to the encapsulation manner; it was mainly due to the
difference in the benzene positions. Figure 8 shows the
Figure 7. Calculated relative Gibbs energies of the stable conformers
at 86 K (ΔG86 K) against CCS of (a) K+(SymDB24C8), (b)
K+(Asym1DB24C8), and (c) K+(Asym2DB24C8). The red and
blue regions indicated the observed CCS area drawn regarding the
experimental ATD measurements in Figure 5.
conformers (up to 10 kJ mol−1) at 86 K (ΔG86 K) against their
CCS values. The red and blue areas in Figure 7 were drawn
based on the experimental CCS distribution with FWHM (∼7
Å2; Figure 5). Regarding K+(SymDB24C8), the stable
conformers could be classified into two groups containing
the calculated CCS values, ∼166 and ∼174 Å2 (Figure 7a). All
conformers were in the red or blue regions of Figure 7a. Thus,
the two maxima observed at 165.3 and 174.4 Å2 of the
experimental CCS curve (Figure 5a) were assigned to the
closed and open conformer groups, respectively, although each
of these conformers could not be separately detected. We also
observed ATD of K+(SymDB24C8) at 300 K (Figure S1,
Supporting Information (SI)). The IM−MS experiment at 300
K could not detect the two components separately, and the
CCS value experimentally determined at 300 K (135.6 0.1
Å2) agrees well with the average value (135.1 Å2) of the stable
conformers whose abundance ratios were determined by the
Boltzmann distribution at 300 K.13,60 Hence, the interconver-
sion between the open and closed conformers occurred at 300
K in K+(SymDB24C8). Figure 7b shows the distribution of the
stable conformers of K+(Asym1DB24C8). The stable con-
formers were distributed much closer than in
K+(SymDB24C8). The separation of the two conformer
groups was not as evident as in K+(SymDB24C8) but these
groups were still detected as two components on the
experimental CCS curve (Figure 5b). Conversely, the CCS
distribution of the stable conformers of K+(Asym2DB24C8)
was located only around ∼173 Å2 (Figure 7c). This coincided
with the experimental ATD result that the ion signal was
distributed only around 173.1 Å2 (Figure 5c).
Figure 8. Distance between the benzene rings in the stable
conformers against CCS for (a) K+(SymDB24C8), (b)
K+(Asym1DB24C8), and (c) K+(Asym2DB24C8). These figures
were drawn for the stable conformers with the ΔG86 K of up to 10 kJ
mol−1.
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benzene−benzene distance in the stable conformers of
K+(DB24C8) against their CCS values. K+(SymDB24C8)
exhibited a clear trend: the smaller the CCS values, the shorter
the benzene−benzene distance. As aforementioned, the CCS
difference between the closed and open conformers was not so
clear in K+(Asym1DB24C8) as it was for K+(SymDB24C8),
although a similar trend was observed between CCS and the
benzene−benzene distance in K+(Asym1DB24C8) (Figure
8b). In K+(Asym2DB24C8), the distribution of CCS was
narrower than that in K+ (SymDB24C8) and
K+(Asym1DB24C8) (Figure 8c). K+(Asym2DB24C8) did
not feature large degrees of freedom in its conformation
because of the narrowed distance between the benzene rings in
the 24C8 cavity.
The spectroscopic results in Figures 3 and 4 availed some
insights into the electronic and geometric structures of
K+(DB24C8). The UVPD spectrum of K+(SymDB24C8)
revealed broad features accompanied by weak, sharp bands.43
The former and latter components in the spectrum were
assigned to the closed and open conformers, respectively.43
The broad features were attributed to the short lifetime of the
excited state related to the formation of the intramolecular
excimer,43 which was confirmed in K+(SymDB24C8) by
fluorescence spectroscopy in solution.43 The coexistence of
the closed and open conformers in the UVPD spectrum at ∼10
K is consistent with the result of IM−MS that the open and
closed conformers were separately detected at 86 K (Figure
5a). This IM−MS result implied that isomerization did not
occur between the open and closed conformers in
K+(SymDB24C8) at 86 or ∼10 K. The intensity of the
Gaussian components in the experimental ATDs directly
represented the relative abundance ratio of the conformers.
The experimental CCS data of K+(SymDB24C8) (Figure 5a)
indicated that the abundance ratio between the closed and
open conformers at 86 K was ∼1:0.65 (S-a and S-b in Figure
5a). This ratio would probably be maintained even at ∼10 K.
The theoretical calculations predicted comparable transition
probabilities for the electronic transitions of the closed and
open conformers (Figure 1d of ref 43). Nevertheless, the band
intensity of the open conformer was substantially weaker than
that of the closed conformer in the UVPD spectrum.
Therefore, this UVPD result for K+(SymDB24C8) suggested
that the relative absorption intensity between the closed and
open conformers was governed by the Franck−Condon factor
(overlap of the vibrational wavefunctions of the electronic
ground and excited states) and not by the relative abundance
ratio.
Figure 9. (a) HOMO and HOMO−1 of the most stable conformers
of K+(SymDB24C8) (isomer sym-a) and (b) K+(Asym1DB24C8)
(isomer asym1-a). The calculations were performed at the M05-2X/
6-311++G(d,p) level.
orbital (HOMO) and second HOMO (HOMO−1) of the
most stable conformer of K+(SymDB24C8) (isomer sym-a).
The molecular orbitals (MOs) were equally delocalized in the
two benzene rings of isomer sym-a.43 This facilitated the
formation of the excimer. Contrarily, MOs were mainly
distributed in one of the benzene rings of the closed conformer
of K+(Asym1DB24C8) (asym1-a, Figure 9b) because of
nonequivalent situations in the two benzene rings: isomer
asym1-a of K+(Asym1DB24C8) possessed a C1 symmetry,
whereas isomer sym-a of K+(SymDB24C8) was of the C2 form.
The distance between the two benzene rings was substantially
longer in asym1-a (4.72 Å) than in sym-a (3.82 Å). This also
prevented the formation of the intramolecular excimer in
K+(Asym1DB24C8). Regarding K+(Asym2DB24C8), the
distance between the benzene rings was rather large (>6 Å)
and was unsuitable for the formation of the intramolecular
excimer. The fluorescence spectrum of K+(Asym2DB24C8)
did not prove the formation of the excimer (Figure 4c). These
geometric features resulted in congested but sharp spectral
features in the UVPD spectrum of K+(Asym2DB24C8).
CONCLUSION
■
As previously suggested, spectroscopic techniques, such as
UVPD, demonstrate a high ability to separately detect different
conformers.28,29,37,39 Since UV transition energy is highly
sensitive to the conformation, UVPD spectroscopy performed
under cold, gas-phase conditions could detect a minute
difference in the conformation as a small difference in the
UV transition energy. However, this spectroscopic technique is
sometimes associated with the challenges of separating the
conformers if they exhibit broad UV absorption due to the
short lifetime of their excited states or spectroscopic
congestion due to many degrees of vibrational freedom. This
study demonstrated that the combination of UVPD spectros-
copy and IM−MS was a powerful tool for resolving conformers
and studying their electronic structures in host-guest
chemistry. The conformations of the K+ complexes of the
three structural isomers of DB24C8, namely
K + ( S y m D B 2 4 C 8 ) , K + ( A s y m 1 D B 2 4 C 8 ) , a n d
K+(Asym2DB24C8), were investigated by UVPD spectrosco-
py, IM−MS in the gas phase, and fluorescence spectroscopy in
methanol solutions. The IM−MS results at 86 K clearly
revealed two components in K+(SymDB24C8) and
In the UVPD spectrum of K+(Asym1DB24C8), broad and
strong features are observed over the entire region of Figure 3b
with a few weak, sharp bands. The IM−MS result (Figure 5b)
implied that there was no interconversion between the closed
and open conformers in K+(Asym1DB24C8) at 86 K and that
the abundance ratio was ∼1:0.55 between the closed and open
conformers (AS1-a and AS1-b in Figure 5b) at 86 K. The
theoretical calculations predicted that the number of closed
conformers was substantially higher than that of the open ones
(Figure 7b). Hence, the broad and sharp components in the
UVPD spectrum of K+(Asym1DB24C8) were ascribed to the
closed and open conformers. The fluorescence spectrum of
K+(Asym1DB24C8) in solution (Figure 4b) did not exhibit
significant Stokes-shifted fluorescence, indicating the absence
of the intramolecular excimer in K+(Asym1DB24C8) upon UV
excitation. Figure 9a shows the highest occupied molecular
H
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K+(Asym1DB24C8), which corresponded to the open and
closed conformers. Further, the IM−MS experiment on
K+(SymDB24C8) at 300 K could not separate the open and
closed conformers because the interconversion between these
conformers occurred at 300 K and not at 86 K. The UVPD
spectrum of K+(Asym1DB24C8) at ∼10 K exhibited broad
features that were accompanied by sharp bands, indicating the
coexistence of several isomers, and this agrees with the IM−
MS result. K+(Asym2DB24C8) exhibited only a set of open
conformations in the CCS experiment. The fluorescence
spectra of these complexes in the methanol solution revealed
that the intramolecular excimer was formed in
K+(SymDB24C8) only. The crown cavity of DB24C8 wrapped
around the K+ ion in K+(DB24C8), although the IM−MS
results availed direct evidence of rapid cavity deformation and
the reconstruction of stable conformers at 300 K. This study
demonstrated that temperature-controlled IM−MS coupled
with the introduction of bulky groups, such as aromatic rings
to host molecules, could reveal the dynamic aspects of
encapsulation in host−guest systems.
Satoru Muramatsu − Department of Chemistry, Graduate
School of Science, Hiroshima University, Higashi-Hiroshima
739-8526, Japan; orcid.org/0000-0002-1575-7570
Manabu Abe − Department of Chemistry, Graduate School of
Science, Hiroshima University, Higashi-Hiroshima 739-8526,
Japan; orcid.org/0000-0002-2013-4394
Takeharu Haino − Department of Chemistry, Graduate
School of Science, Hiroshima University, Higashi-Hiroshima
739-8526, Japan; orcid.org/0000-0002-0945-2893
Takayuki Ebata − Department of Chemistry, Graduate School
of Science, Hiroshima University, Higashi-Hiroshima 739-
8526, Japan; orcid.org/0000-0002-6236-9116
Fuminori Misaizu − Department of Chemistry, Graduate
School of Science, Tohoku University, Sendai 980-8578,
Japan; orcid.org/0000-0003-0822-6285
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jpca.0c09068
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
■
ACKNOWLEDGMENTS
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jpca.0c09068.
This work was supported by JSPS KAKENHI (Grant Nos.
JP16H04098, JP20H00374 (Y.I.), and JP16K05641 (K.O.)),
the Institute for Quantum Chemical Exploration, and The Salt
Science Research Foundation (Grant No. 1916). Some of the
calculations were conducted at the Research Center for
Computational Science, Okazaki, Japan.
Relative Gibbs energies, theoretical CCSs, benzene−
benzene distances and the average distance between the
K+ ion and O atoms of the stable conformers of
K+(DB24C8) at 86 K, the observed and simulated ATDs
of K+(SymDB24C8) at 300 K, the relative Gibbs
energies and theoretical CCSs of the stable conformers
of K+(SymDB24C8) at 300 K, geometrical coordinates
of the conformers in Figure 6, and a full list of the
authors of refs 12 and 55 (PDF)
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AUTHOR INFORMATION
■
Corresponding Authors
Keijiro Ohshimo − Department of Chemistry, Graduate
School of Science, Tohoku University, Sendai 980-8578,
Japan; orcid.org/0000-0002-2397-6896;
Email: keijiro.ohshimo.d1@tohoku.ac.jp
Yoshiya Inokuchi − Department of Chemistry, Graduate
School of Science, Hiroshima University, Higashi-Hiroshima
739-8526, Japan; orcid.org/0000-0001-7959-5315;
Email: y-inokuchi@hiroshima-u.ac.jp
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Sota Tainaka − Department of Chemistry, Graduate School of
Science, Tohoku University, Sendai 980-8578, Japan
Tomoyuki Ujihira − Department of Chemistry, Graduate
School of Science, Hiroshima University, Higashi-Hiroshima
739-8526, Japan
Mayuko Kubo − Department of Chemistry, Graduate School
of Science, Hiroshima University, Higashi-Hiroshima 739-
8526, Japan
Motoki Kida − Department of Chemistry, Graduate School of
Science, Hiroshima University, Higashi-Hiroshima 739-8526,
Japan
Daisuke Shimoyama − Department of Chemistry, Graduate
School of Science, Hiroshima University, Higashi-Hiroshima
739-8526, Japan
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