UV and IR Spectroscopy of Transition MetalâCrown Ether Complexes

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

UV and IR Spectroscopy of Transition Metal–Crown Ether Complexes in the Gas Phase: Mn (Benzo-15-Crown-5)(HO) 2+

2

0–2

Yoshiya Inokuchi, Takayuki Ebata, and Thomas R. Rizzo J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b05706 • Publication Date (Web): 12 Jul 2019 Downloaded from pubs.acs.org on July 24, 2019

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The Journal of Physical Chemistry

UV and IR Spectroscopy of Transition Metal–Crown Ether Complexes in the Gas Phase: Mn2+(benzo-15-crown-5)(H2O)0–2

Yoshiya Inokuchi,‡,* Takayuki Ebata,‡ and Thomas R. Rizzo† Department of Chemistry, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan and Laboratoire de Chimie Physique Moléculaire, École Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland E-mail: [email protected] Phone: +81(Japan)-82-424-7407

Abstract Ultraviolet spectroscopy

are

photodissociation performed

for

(UVPD)

bare

and

and

IR-UV

micro-hydrated

double-resonance complexes

of

Mn2+(benzo-15-crown-5), Mn2+(B15C5)(H2O)n (n = 0–2), under cold gas-phase conditions.

Density functional theory (DFT) calculations are also carried out to derive

information on the geometric and electronic structures of the complexes from the experimental results.

The n = 0 complex shows broad features in the UVPD spectrum,

whereas the UV spectra of the n = 1 and 2 complexes exhibit sharp vibronic bands. The IR-UV and DFT results suggest that there is only one isomer each for the n = 1 and 2 complexes in which H2O molecules are directly attached to the Mn2+ ion through Mn2+•••OH2 bonds with no intermolecular bond between the water molecules.

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Time-dependent DFT (TD-DFT) calculations suggest that the π–π* transition of the B15C5 part is highly mixed with the “ligand to metal charge transfer (LMCT)” transition in the n = 0 complex, which can result in broad features in the UVPD spectrum.

In contrast, attachment of H2O molecules to Mn2+(B15C5) suppresses the

mixing, providing sharp vibronic bands assignable to the π–π* transition for the n = 1 and 2 complexes.

These results indicate that the electronic structure and transition of

benzo-crown ether complexes with transition metals are strongly affected by solvation.

*To whom correspondence should be addressed. ‡Hiroshima †École

University

Polytechnique Fédérale de Lausanne


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1. Introduction Since their discovery by Pedersen, crown ethers (CEs) have been extensively used as phase transfer catalysis and building blocks in organic and supramolecular chemistry.1-3

Pedersen initially focused his attention primarily on CE complexes with

alkali and alkaline earth metal ions, where the complexes are formed by electrostatic interaction between metal cations and negatively charged oxygen atoms of the CEs.1-2 Ultraviolet spectra of CE complexes with some transition metal ions in solution have also been reported.2

However, the UV spectra were broad and uninformative with

respect to the geometric and electronic structure of the complexes. studied the interactions of CEs with transition metal cations.4

Su and Weiher

They proposed probable

structures of Co2+ complexes with CEs on the basis of spectral (visible and far infrared) and

magnetic

data.

The

spectral

data

suggested

the

existence

of

a

CoCl42– counter-anion in the solid product, but no information was obtained for the cationic (Co2+–CE) part.4

Subsequently, a number of papers have reported CE

complexes with transition metal ions in the condensed phase and the gas phase.5-19 Concerning spectroscopy of divalent transition-metal ion–CE complexes in the gas phase,

Rodriguez

and

Lisy

reported

IR

predissociation

spectroscopy

of

Mn2+(18-crown-6)(CH3OH)n (n = 1–3) complexes in the CH and OH stretching regions.20

In the n = 2 and 3 complexes, a CH3OH•••CH3OH intermolecular bond is

formed, providing intense hydrogen-bonded OH stretching bands.

Cooper et al.

performed IR multiple photon spectroscopy of Zn2+ and Cd2+ complexes with CEs using a free electron laser21 and determined the conformation of the CE part on the basis of the IR spectra in the 750–1600 cm–1 region.

We have been studying the geometric and

electronic structure of benzo-CE complexes with alkali and alkaline earth metal ions by



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UV photodissociation (UVPD), IR-UV double-resonance, and UV-UV hole-burning spectroscopy.22-26 In the present study, we report UV and IR spectroscopy of bare and micro-hydrated Mn2+ complexes with benzo-15-crown-5 (B15C5), Mn2+(B15C5)(H2O)n (n = 0–2), in the gas phase as the first step towards developing a molecular-level understanding of the geometric and electronic structure for transition metal–CE complexes.

We use a tandem mass spectrometer equipped with an electrospray ion

source and a cryogenic, 22-pole ion trap to perform UVPD and IR-UV double-resonance spectroscopy and analyze the data with the aid of quantum chemical calculations.

We discuss the effect of micro-hydration on the geometric and electronic

structure of the Mn2+(B15C5) complex as a function of the number of H2O molecules. 2. Experimental and Computational Methods The details of our experimental approach have been given elsewhere.22, 27-30 Briefly, the Mn2+(B15C5)(H2O)n (n = 0–2) complexes are produced continuously at atmospheric pressure via nanoelectrospray of a solution containing MnCl2 and B15C5 (~10 µM each) dissolved in methanol/water (~9:1 volume ratio).

The parent ions of

interest are mass-selected in a quadrupole mass filter and introduced into a 22-pole RF ion trap, which is cooled by a closed-cycle He refrigerator to 6 K.

The trapped ions

are cooled internally and translationally to ~10 K through collisions with cold He buffer gas,22, 27, 31-32 which is pulsed into the trap.

The trapped ions are then irradiated with a

UV laser pulse, which causes some fraction of them to dissociate.

The resulting

charged photofragments, as well as the remaining parent ions, are released from the trap, mass-analyzed by a second quadrupole, and detected with a channeltron electron multiplier.

UVPD spectra of parent ions are obtained by plotting the yield of the


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photofragment ion as a function of the UV laser wavenumber.

The UVPD spectra of

the Mn2+(B15C5)(H2O)n (n = 0–2) complexes are measured by monitoring the yield of Mn2+ ion for n = 0 and Mn2+(B15C5) ion for n = 1 and 2.

For IR-UV

double-resonance spectroscopy, the output pulse of an IR OPO precedes the UV pulse by ~100 ns and counter-propagates collinearly with it through the 22-pole trap. Absorption of the IR light by the ions warms them up, modifying their UV absorption.33 We obtain IR-UV depletion spectra by fixing the wavenumber of the UV laser to a vibronic transition of a specific conformer and then scanning the wavenumber of the IR OPO. We also perform density functional theory (DFT) calculations to determine the geometric and electronic structure of the complexes.

For geometry optimization of

the Mn2+(B15C5)(H2O)n (n = 0–2) complexes, we firstly use a classical force field to find conformational minima.

The initial conformational search is performed with the

CONFLEX High Performance Conformation Analysis program with the MMFF94s force field.29, 34-36

Minimum-energy conformers found with the force field calculations

are then optimized at the M06/6-311++G(d,p) level using the GAUSSIAN09 program package.37

We use the M06 functional for the Mn2+(B15C5)(H2O)n complexes

because M06 includes parameters suitable for transition metals; the M06-2X functional was not recommended for transition metals.38-40

Vibrational analysis is carried out for

the optimized structures at the M06/6-311++G(d,p) level.

Calculated frequencies are

scaled with a factor of 0.94 for comparison with the IR-UV spectra.

This factor is

determined so as to simulate the frequency of the OH stretching vibrations for H2O in the gas phase.41

We estimate the contribution of the Mn and B15C5 parts to molecular

orbitals (MOs) using the GaussSum ver. 3.0 written by O’Boyle et al.42

We perform

time-dependent DFT (TD-DFT) calculations at the M06/6-311++G(d,p) level to



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determine the electronic transition energy, oscillator strength, coefficients in the configuration interaction (CI) expansion of the electronic transition, and geometry in the excited state.

The contribution of the Mn2+ and B15C5 parts to the electronic

transitions is derived from the results of MOs and CI. 3. Results and Discussion Figure 1 displays the UVPD spectra of the Mn2+(B15C5)(H2O)n (n = 0–2) complexes in the 37000–37500 cm–1 region.

The UVPD spectrum of the n = 1

complex (Figure 1b) is reproduced from our previous study.29 absorption in this region.

All the complexes show

Since the UVPD spectrum of the Na+(B15C5) complex

shows the origin band of the S1–S0 transition at 36555 cm–1,23 the UV absorption of the Mn2+(B15C5) complexes is higher in energy than that of Na+(B15C5).

The UV

spectrum of bare Mn2+(B15C5) (Figure 1a) is broad and unresolved, while the n = 1 and 2 complexes have sharp vibronic bands around 37200 cm–1. the hydrated complexes is different between n = 1 and 2.

The vibronic structure of

The UVPD spectrum of the

n = 1 complex (Figure 1b) shows an extensive progression with an interval of ~34 cm–1. The band at 37082 cm–1, which is assignable to the origin band, is very weak.

These

results suggest that the geometry undergoes a large change upon the electronic excitation for the n = 1 complex.

In addition, there is a strong band at 37218 cm–1.

In the UVPD spectrum of the n = 2 complex, a strong origin band is observed at 37199 cm–1 with two additional bands with an interval of ~27 cm–1, indicating a small change of the geometry upon electronic excitation. Figure 2 displays the IR-UV spectra (red curves) of the n = 1 and 2 complexes in the OH stretching (3400–3800 cm–1) region.

The IR-UV spectrum of the n = 1

complex (red curve in Figure 2a) is taken from our previous study.29


We include

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additional IR-UV data of the n = 1 and 2 complexes in Figures S1 and S2 of the Supporting Information.

All the vibronic bands of the n = 1 complex in Figure 1b

show a depletion of the intensity under the IR irradiation at 3609 and 3684 cm–1 (Figure S1), suggesting that all the vibronic bands in the UVPD spectrum of the n = 1 complex have the same IR spectrum.

We have reported IR-UV spectra of B15C5 complexes

with divalent metal ions solvated with an H2O molecule, M2+(B15C5)(H2O) (M = Ca, Sr, Ba, Mn).29 In these ions, the H2O molecule is bonded to the M2+ ion directly through the M2+•••OH2 intermolecular bonds, which provides the symmetric and asymmetric OH stretching vibrations of the free OH groups. Thanks to the cooling ions in the cold ion trap, these OH bands are very sharp, with a FWHM of less than 3 cm–1. As a result, it was possible to distinguish different isomers by IR-UV spectroscopy with the difference in the frequency of the OH stretching vibrations of only ~1.0 cm–1. In the case of the Mn2+(B15C5)(H2O) complex, the IR-UV spectrum shows band widths of 4.4 and 2.8 cm–1 (FWHM) for the symmetric and asymmetric OH stretching vibrations, respectively (Figure 2a). It seems hardly plausible that different isomers have the same free OH frequency by coincidence with an accuracy of ~1 cm–1 for both of the symmetric and asymmetric OH stretching vibrations. Hence, it is reasonable that IR-UV hole-burning spectroscopy (Figure S1) provides evidence that the Mn2+(B15C5)(H2O) complex has one dominant isomer under cold, gas-phase conditions. In the case of the n = 2 complex, the two strong UVPD bands in Figure 1c show the same IR-UV spectra (Figure S2).

The third vibronic band of the n = 2

complex at 37253 cm–1 can be included in the regular progression with an interval of 27 cm–1 (Figure 1c).

Hence, we conclude that we observe only one conformer also for the

n = 2 complex.



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Figure 3 shows the most and the second most stable structures of the Mn2+(B15C5)(H2O)0–2 complexes calculated at the M06/6-311++G(d,p) level.

In all

the complexes, the B15C5 part opens its cavity the most, and the Mn2+ ion is located almost at the center of the cavity.

These complexes have the sextet (highest) spin state,

and the complexes with lower spin states are higher than the sextet complexes by more than 250 kJ mol–1.

For the hydrated complexes, the conformation of the Mn2+(B15C5)

part is similar to that of the bare one (Figure 3a).

In the n = 1 complex, two stable

isomers are obtained in the calculations (MnB15C5W1-I and -II, Figure 3b).

The

water molecule is bonded directly to the Mn2+ ion but on different sides of the Mn2+(B15C5) part in these isomers.

The most stable form of the n = 2 complex

(MnB15C5W2-I, Figure 3c) has one H2O molecule on opposite sides of the Mn2+(B15C5) part.

Also in the second most stable isomer (MnB15C5W2-II, Figure

3c), the two H2O molecules are independently bonded to the Mn2+ ion. The IR spectra calculated for the most stable isomers of the n = 1 and 2 complexes are shown with black bars in Figure 2. intensity are also displayed in Table 1.

The calculated frequency and IR

The most stable isomer of the n = 1 complex

(MnB15C5W1-I, Figure 3b) has two IR bands at 3591 and 3683 cm–1; these bands are attributed to the symmetric and asymmetric OH stretching vibrations, respectively. The IR band position in the calculated spectrum is similar to that in the IR-UV spectrum (3609 and 3684 cm–1).

As shown in Figure S3 of the Supporting Information, the most

and the second most stable isomers of the n = 1 complex (MnB15C5W1-I and -II, Figure 3b) show similar IR spectra to each other; it is not possible to determine the structure of the n = 1 complex definitely on the basis of the IR spectra. MnB15C5W1-I is more stable than MnB15C5W1-II by 2.1 kJ mol–1.

Isomer

Hence, we

attribute the structure of the n = 1 complex to the most stable isomer (Figure 3b).


In

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the case of the n = 2 complex, the calculated IR frequency of the most stable isomer (MnB15C5W2-I, Figure 3c) is 3602, 3604, 3699, and 3699 cm–1.

The former two

bands are the symmetric OH stretching vibrations, and the latter ones are the asymmetric OH stretching vibrations; the frequency of the symmetric OH bands are slightly different between the two bands, whereas the asymmetric ones have almost the same frequency.

This calculated spectrum resembles the IR-UV spectrum very well;

the vibrational bands around 3615 cm–1 show two resolved features at 3614 and 3618 cm–1, but only one peak is observed at 3698 cm–1.

The most and the second most

stable isomers of the n = 2 complex have similar IR spectra (Figure S3), but the difference in the total energy between them is substantial (17 kJ mol–1, Figure 3c). Hence, we ascribe the structure of the n = 2 complex in the experiment to the most stable isomer (MnB15C5W2-I, Figure 3c). We examine the origin of different spectral features between the UVPD spectra of the complexes (Figure 1) by TD-DFT calculations. calculated

electronic

transitions

for

the

most

Mn2+(B15C5)(H2O)0–2 and Na+(B15C5) complexes.

stable

Figure 4 displays the isomers

of

the

In our previous studies on

benzo-CE complexes with alkali metal ions, we found that TD-DFT calculations tend to overestimate the electronic transition energy; scaling factors (~0.834) were employed for the calculated transition energy to compare calculated electronic spectra with UVPD spectra. However, the TD-DFT calculations well reproduce relative positions of the UV absorption among the benzo-CE complexes with alkali metal ions.22-26 In the UVPD spectrum of the Na+(B15C5) complex, the band origin is observed at 36555 cm–1.23 The electronic transition of the Mn2+(B15C5) complexes in the UVPD spectra is substantially higher in energy (~37100 cm–1, Figure 1) than that of the Na+(B15C5) complex. The energy of the S1–S0 transition for the Na+(B15C5) complex was



estimated as 4.83 eV (Figure 4d).

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The Mn2+(B15C5)(H2O)0–2 complexes do not have

strong absorption around 4.83 eV, but some strong transitions are found on the higher energy side; this is similar to a trend of the UVPD results.

Hence, we ascribe the

UVPD spectra of the Mn2+(B15C5)(H2O)n (n = 0–2) complexes to states 24, 17 and 14, respectively, which are located on the higher energy side of the strong transition of the Na+(B15C5) complex (Figure 4). MOs of the Mn2+(B15C5)(H2O)0–2 complexes contributing to the electronic transitions below 6 eV are almost localized either on the The electronic transitions of the Mn2+(B15C5)(H2O)0–2

B15C5 part or the Mn atom.

complexes can be ascribed either to the ligand to metal charge transfer (LMCT), to the local excitation (LE), or to the mixture of them.

The LMCT transitions correspond to

electron promotion from an occupied π orbital of B15C5 to a d orbital of Mn.

The LE

is mainly the π–π* transition of the B15C5 part for the Mn2+(B15C5)(H2O)n complexes. Table 2 shows the calculated electronic transition energy, oscillator strength, and contribution of the LMCT and LE to each electronic transition.

In the case of the

Mn2+(B15C5) complex, there are two strong bands assignable to the LMCT below 4 eV; these bands are located out of our observation range in the present study.

In the

5.0–5.5 eV region, there are four electronic transitions (excited states 20 and 23–25, Figure 4a) with comparable intensity.

The electronic transitions to these states have a

mixed nature of the LE(π–π*) and LMCT (Table 2).

The UV absorption of the

Mn2+(B15C5) complex in Figure 1a can be ascribed mainly to the electronic transition to state 24, because the transition to this state has the highest oscillator strength among the four electronic transitions.

However, the existence of low-lying excited states

having a similar (π, π*) character (states 20 and 23) can promote the non-radiative decay or internal conversion from state 24 to 20 and 23, providing short lifetimes and broad spectral features (Figure 1a).

In the case of the Mn2+(B15C5)(H2O)1,


2

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complexes, there is one strong transition around 5.2 eV (excited state 17 and 14) for n = 1 and 2, respectively (see Figures 4b and c), and these transitions are responsible for the observed UVPD spectra in Figures 1b and c.

These transitions have a main

contribution from the LE(π–π*) (> ~80%, Table 2); the strong electronic transition of the n = 1 and 2 complexes in the UV region keeps its π–π* nature as alkali metal ion–B15C5 complexes.23 The strong mixing of the LE(π–π*) with the LMCT for the Mn2+(B15C5) complex will result in large structural change between the ground and excited states. We perform geometry optimization of the Mn2+(B15C5)(H2O)n (n = 0–2) complexes in excited states 24, 17, and 14, respectively (see Figure 4).

As shown in Figures 1b and

c, the UVPD spectra of the n = 1 and 2 complexes show a low-frequency progression. Our previous studies on UVPD spectroscopy of alkali metal ion–benzo-CE complexes suggest that low-frequency progressions in the UV spectra are ascribed to normal modes that change dihedral angles around the catechol part, namely, the dihedral angle of C1–O2–C3–C4 in Figure 3a.22-24, 43

The dihedral angle of the stable structures in

the ground and excited states (black and red numbers, respectively) is shown in Figure 3. In the case of the n = 1 complex (Figure 3b), the difference of the dihedral angle is 4.2˚. In contrast, the difference is only 1.0˚ for the n = 2 complex (Figure 3c), smaller than that of the n = 1 complex.

These calculation results agree with a trend of the UVPD

spectra that the n = 2 complex shows a strong origin band, whereas the UVPD spectrum of the n = 1 complex has a weak origin band with an extensive low-frequency progression.

We also performed geometry optimization of the Mn2+(B15C5) complex.

However, geometry optimization in the electronic excited state did not provide a stable structure from the stable geometry in the ground state after a long period of calculation time (more than ten times longer than that for the hydrated complexes).


This


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calculation suggests that the structural change upon the UV excitation is substantial for the Mn2+(B15C5) complex.

This also can be the origin of the broad features in the

UVPD spectrum of the Mn2+(B15C5) complex. 4. Conclusion We have observed the UVPD and IR-UV double-resonance spectra of the Mn2+(B15C5)(H2O)n (n = 0–2) complexes under cold (~10 K), gas-phase conditions. The Mn2+(B15C5) complex shows broad spectral features in the UVPD spectrum, while the UVPD spectra of the Mn2+(B15C5)(H2O)1, 2 complexes consist of resolved, sharp The IR-UV results of the Mn2+(B15C5)(H2O)1, 2 complexes in the OH

vibronic bands.

stretching region indicate that the hydrated complexes have only one isomer each, and that the H2O molecules in the complexes are bonded directly to the Mn2+ ion.

The

TD-DFT results of the Mn2+(B15C5) complex suggest that the π–π* transition is strongly mixed with transitions of the LMCT, which can provide broad features in the UVPD spectrum.

Acknowledgment This work is partly supported by JSPS KAKENHI (grant number JP16H04098).

YI and TE thank the support from JSPS through the program “Strategic

Young Researcher Overseas Visits Program for Accelerating Brain Circulation”.

A

part of the calculations is performed using Research Center for Computational Science, Okazaki, Japan.

The work was also supported by the Swiss National Science

Foundation (grant number 200020-165908).


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Supporting Information Available: additional results of IR-UV spectroscopy and vibrational analysis by DFT calculations for the Mn2+(B15C5)(H2O)n (n = 0–2) complexes.

Full author list of ref. 37.



Table 1. Calculated frequency (cm–1) and IR intensity (km mol–1) of the OH stretching vibrations of the Mn2+(B15C5)(H2O)1, 2 complexes at the M06/6-311++G(d,p) level. Frequency of the OH stretching vibrations (cm–1) observed in the IR-UV spectra of the Mn2+(B15C5)(H2O)1, 2 complexes. Calculated Frequencya (cm–1)

IR Intensity Observed Frequency (km mol–1) (cm–1) 2+ Mn (B15C5)(H2O)1 3591 141 3609 3683 198 3684 Mn2+(B15C5)(H2O)2 3602 137 3614 3604 60 3618 3699 174 3698 3699 149 3698 aA scaling factor of 0.94 is employed for the calculated frequencies. It is determined so as to reproduce the frequency of the OH stretching vibrations of H2O in the gas phase.


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Table 2. Calculated electronic transition energy (eV), oscillator strength, and contribution from the LE (mainly π–π* transition) and LMCT to each electronic transition. State

Transition Energy (eV)

4 6 20 23 24 25

3.30 3.85 5.01 5.17 5.26 5.35

3 11 17 18

3.93 4.67 5.22 5.32

6 14 17

4.63 5.21 5.37

Oscillator Strength

Mn2+(B15C5) 0.0102 0.0103 0.0055 0.0033 0.0176 0.0073 Mn2+(B15C5)(H2O)1 0.0105 0.0090 0.0249 0.0036 Mn2+(B15C5)(H2O)2 0.0061 0.0219 0.0106


LE

LMCT

0.00 0.01 0.21 0.41 0.92 0.88

0.98 0.97 0.75 0.52 0.04 0.07

0.03 0.11 0.90 0.86

0.95 0.87 0.07 0.09

0.10 0.80 0.57

0.85 0.17 0.40


Figure 1. UVPD spectra of the Mn2+(B15C5)(H2O)n (n = 0–2) complexes. The spectrum of the n = 1 complex (panel b) was reproduced from the previous study (ref. 29). The arrows in the figure represent the position at which IR-UV spectra are measured (see Figure 2).


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Figure 2. IR-UV (red curves) and theoretical IR (black bars) spectra of the Mn2+(B15C5)(H2O)n (n = 1, 2) complexes in the OH stretching region. The spectrum of the n = 1 complex (panel a) was reproduced from the previous study (ref. 29). A scaling factor (0.94) was employed for the calculated frequencies. The UV frequency at which the intensity of fragment ions is monitored for the IR-UV spectra is shown with arrows in Figure 1. Numbers and italic numbers in the figure represent the position of the depletion and the calculated frequencies, respectively.



Figure 3. Stable structures of the Mn2+(B15C5)(H2O)n complexes with (a) n = 0, (b) n = 1, and (c) n = 2 determined at the M06/6-311++G(d,p) level. Numbers in parentheses represent the relative total energy in kJ mol–1 to that of the most stable isomer. Numbers next to the structure are dihedral angles of C1–O2–C3–C4 (see Figure 3a) in the ground state (black) and excited state (red) determined by geometry optimization.


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Figure 4. Calculated electronic transitions of the most stable isomers for (a–c) Mn2+(B15C5)(H2O)n (n = 0–2) and (d) Na+(B15C5) complexes at the M06/6-311++G(d,p) level. Numbers in the figure represent the label of the electronic excited states; “1” means the first excited state, and so on.



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UV and IR Spectroscopy of Transition MetalâCrown Ether Complexes

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