We measure UV photodissociation (UVPD) and UV–UV hole-burning (HB) spectra of dibenzo-15-crown-5 (DB15C5) complexes with alkali metal ions, M+·DB15C5 (M = Li, Na, K, Rb, and Cs), under cold (∼10 K) conditions in the gas phase. The UV–UV HB spectra of the M+·DB15C5 (M = K, Rb, and Cs) complexes indicate that there is one dominant conformation for each complex except the Na+·DB15C5 complex, which has two conformers with a comparable abundance ratio. It was previously reported that the M+·(benzo-15-crown-5) (M+·B15C5, M = K, Rb, and Cs) complexes each have three conformers. Thus, the attachment of one additional benzene ring to the crown cavity of benzo-15-crown-5 reduces conformational flexibility, giving one dominant conformation for the M+·DB15C5 (M = K, Rb, and Cs) complexes. In the UVPD spectra of the K+·DB15C5, Rb+·DB15C5, and Cs+·DB15C5 complexes, the S1–S0 and S2–S0 transitions are observed independently at different positions with different vibronic structures. The spectral features are substantially different from those of the K+·(dibenzo-18-crown-6) (K+·DB18C6) complex, which belongs to the C2v point group and exhibits exciton splitting with an interval of 2.7 cm–1. The experimental and theoretical results suggest that in the M+·DB15C5 complexes the two benzene rings are not symmetrically equivalent with each other and the S1–S0 and S2–S0 electronic excitations are almost localized in one of the benzene rings. The electronic interaction energy between the two benzene chromophores is compared between the K+·DB15C5 and K+·DB18C6 complexes by quantum chemical calculations. The interaction energy of the K+·DB15C5 complex is estimated to be less than half of that of the K+·DB18C6 complex (∼30 cm–1) due to less suitable relative angles between the transition dipole moments of the two benzene chromophores in K+·DB15C5.

We measure UV and IR spectra in the gas phase for EuOH+, EuCl+, and TbO+ ions, which are produced by an electrospray ionization source and cooled to ∼10 K in a cold, 22-pole ion trap. The UV photodissociation (UVPD) spectra of these ions show a number of sharp, well-resolved bands in the 30000–38000 cm–1 region, although a definite assignment of the spectra is difficult because of a high degree of congestion. We also measure an IR spectrum of the EuOH+ ion in the 3500–3800 cm–1 region by IR–UV double-resonance spectroscopy, which reveals an OH stretching band at 3732 cm–1. We perform density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations of these ions in order to examine the nature of the transitions. The DFT results indicate that the states of highest-spin multiplicity (octet for EuOH+ and EuCl+ and septet for TbO+) are substantially more stable than other states of lower-spin multiplicity. The TD-DFT calculations suggest that UV absorption of the EuOH+ and EuCl+ ions arises from Eu(4f) → Eu(5d,6p) transitions, whereas electronic transitions of the TbO+ ion are mainly due to the electron promotion of O(2p) → Tb(4f,6s). The UVPD results of the lanthanide-containing ions in this study suggest the possibility of using lanthanide ions as “conformation reporters” for gas-phase spectroscopy for large molecules.

The ultraviolet photodissociation (UVPD) spectra of calix[4]arene (C4A) complexes with alkali metal ions, M+·C4A (M = Na, K, Rb, and Cs), are measured in the 34 000–37 000 cm−1 region under cold (∼10 K) conditions in the gas phase. The UVPD spectra of the Na+·C4A and K+·C4A complexes show several sharp vibronic bands, while the UVPD spectra of the Rb+·C4A and Cs+·C4A complexes exhibit only broad features. The UVPD spectra are assigned with the aid of quantum chemical calculations. Most of the features in the UVPD spectra can be attributed to cone isomers, which are the most stable for all the M+·C4A complexes. In all the cone isomers, the M+ ion is encapsulated inside the cavity of C4A, and the structure is distorted to C2 symmetry from that of bare C4A (C4 symmetry). The cone isomers show a big difference in the electronic structure between the K+ and Rb+ complexes. The Rb+ and Cs+ complexes have an electronic structure similar to that of bare C4A. In the Na+ and K+ complexes, the two benzene rings facing each other in a pair have a short distance between them (<6 Å). This results in a substantial overlap of the π clouds between them, and an electronic transition is localized on this pair. Only this localized electronic transition of the Na+ and K+ complexes shows sharp band features in the UVPD spectra. In the Na+·C4A complex, the UVPD spectroscopic results suggest the coexistence of other isomers having partial-cone and 1,3-alternate forms. The energetics of the isomerization reactions of C4A and Na+·C4A is examined theoretically. The estimated potential barriers between the stable conformers are less than 75 kJ mol−1 for Na+·C4A, suggesting that conformational conversion can occur at room temperature, before the Na+·C4A complex enters the cold ion trap. The existence of multiple conformations for Na+·C4A is attributed to the higher stability of these conformers, both kinetically and thermodynamically, compared to the case of bare C4A and the other M+·C4A complexes.

We measure UV photodissociation (UVPD) spectra of benzo-12-crown-4 (B12C4) complexes with alkali metal ions, M+·B12C4 (M = Li, Na, K, Rb, and Cs), in the 36300–37600 cm–1 region. Thanks to the cooling of ions to ∼10 K, all the M+·B12C4 complexes show sharp vibronic bands in this region. For UV–UV hole-burning (HB) spectroscopy, we first check if our experimental system works well by observing UV–UV HB spectra of the K+ complex with benzo-18-crown-6 (B18C6), K+·B18C6. In the UV–UV HB spectra of the K+·B18C6 complex, gain signals are also observed; these are due to vibrationally hot K+·B18C6 complex produced by the UV excitation of cold K+·B18C6 complex. Then we apply UV–UV HB spectroscopy to the M+·B12C4 complexes, and only one conformer is found for each complex except for the Li+ complex, which has two conformers. The vibronic structure around the origin band of the UVPD spectra is quite similar for all the complexes, indicating close resemblance of the complex structure. The most stable structures calculated for the M+·B12C4 (M = Li, Na, K, Rb, and Cs) complexes also have a similar conformation among them, which coincides with the UVPD results. In these conformers the metal ions are too big to be included in the B12C4 cavity, even for the Li+ ion. In solution, it was reported that 12-crown-4 (12C4) shows the preference of Na+ ion among alkali metal ions. From the similarity of the structure for the M+·B12C4 complexes, it is suggested that the solvation of free metal ions, not of the M+·12C4 complexes, may lead to the selectivity of Na+ ion for 12C4 in solution.

We demonstrate a powerful spectroscopic technique, surface-enhanced infrared absorption (SEIRA) spectroscopy, not only for detecting host–guest complexes in solution but also for examining the relationship between the guest selectivity, complex structure, and solvent effect. We synthesize thiol derivatives of 15-crown-5 and 18-crown-6 [2-(6-mercaptohexyloxy)methyl-15-crown-5 (15C5-C1OC6-SH) and 2-(6-mercaptohexyloxy)methyl-18-crown-6 (18C6-C1OC6-SH)], which are adsorbed on gold surfaces through S–Au bonds. The IR difference spectra of the M+·15C5-C1OC6 (M = Li, Na, K, Rb, and Cs) complexes on gold are observed using aqueous solutions of MCl by SEIRA spectroscopy. The spectra show a noticeable change in the C–O stretching vibration at around 1100 cm−1. The spectral patterns of M+·15C5-C1OC6 are similar for Li+ and Na+, and for K+, Rb+, and Cs+; the interaction between the metal ions and 15C5-C1OC6 changes drastically between Na+ and K+ in the series of alkali metal ions. On the other hand, the equilibrium constant for complex formation determined by the IR intensity shows clear preference for Na+ ions. We also observe the IR difference spectra of M+·18C6-C1OC6 in methanol and compare them with those in water. The spectral patterns in methanol are almost the same as those in water, but the equilibrium constant in methanol does not show preference for any ion, different from the K+ preference in water. From these findings we attribute the origin of the ion selectivity of 15C5 and 18C6 in solution to the interaction between the metal ions and the crown ethers in the complexes or the solvation energy of free ions. In the case of 15C5-C1OC6 in water, the preference of Na+ over K+, Rb+, and Cs+ can be attributed to the strength of the interaction or the size matching between metal ions and 15C5-C1OC6; the Na+ selectivity over Li+ ions is dominated by the solvation energy of free ions. For 18C6-C1OC6 in methanol, the equilibrium constant for complex formation becomes much bigger in methanol than that in water and loses the selectivity in methanol, because the solvation energy in methanol is fairly smaller than that in water, predominating the contribution from the strength of the interaction between metal ions and 18C6-C1OC6. The IR spectra measured by SEIRA spectroscopy are quite sensitive to properties of host–guest complexes such as the intermolecular interaction, the structure, and the orientation against the gold surface. However, the evidence for guest selectivity emerges primarily in the intensity of the spectra, rather than band positions or spectral patterns in the IR spectra.

The cooling of ionic species in the gas phase greatly simplifies the UV spectrum, which is of special importance when studying the electronic and geometric structures of large systems, such as biorelated molecules and host–guest complexes. Many efforts have been devoted to achieving ion cooling with a cold, quadrupole Paul ion trap (QIT), but one problem was the insufficient cooling of ions (up to ∼30 K) in the QIT. In this study, we construct a mass spectrometer for the ultraviolet photodissociation (UVPD) spectroscopy of gas-phase cold ions. The instrument consists of an electrospray ion source, a QIT cooled with a He cryostat, and a time-of-flight mass spectrometer. With great care given to the cooling condition, we can achieve ∼10 K for the vibrational temperature of ions in the QIT, which is estimated from UVPD spectra of the benzo-18-crown-6 (B18C6) complex with a potassium ion, K+·B18C6. Using this setup, we measure a UVPD spectrum of cold calix[4]arene (C4A) complex with potassium ion, K+·C4A. The spectrum shows a very weak band and a strong one at 36018 and 36156 cm–1, respectively, accompanied by many sharp vibronic bands in the 36000–36600 cm–1 region. In the geometry optimization of the K+·C4A complex, we obtain three stable isomers: one endo and two exo forms. On the basis of the total energy and UV spectral patterns predicted by density functional theory calculations, we attribute the structure of the K+·C4A complex to the endo isomer (C2 symmetry), in which the K+ ion is located inside the cup of C4A. The vibronic bands of K+·C4A at 36 018 and 36 156 cm–1 are assigned to the S1(A)–S0(A) and S2(B)–S0(A) transitions of the endo isomer, respectively.

The H2O+ radical ion, produced in an electrospray ion source via charge transfer from Eu3+, is encapsulated in benzo-15-crown-5 (B15C5) or benzo-18-crown-6 (B18C6). We measure UV photodissociation (UVPD) spectra of the (H2O·B15C5)+ and (H2O·B18C6)+ complexes in a cold, 22-pole ion trap. These complexes show sharp vibronic bands in the 35 700–37 600 cm–1 region, similar to the case of neutral B15C5 or B18C6. These results indicate that the positive charge in the complexes is localized on H2O, giving the forms H2O+·B15C5 and H2O+·B18C6, in spite of the fact that the ionization energy of B15C5 and B18C6 is lower than that of H2O. The formation of the H2O+ complexes and the suppression of the H3O+ production through the reaction of H2O+ and H2O can be attributed to the encapsulation of hydrated Eu3+ clusters by B15C5 and B18C6. On the contrary, the main fragment ions subsequent to the UV excitation of these complexes are B15C5+ and B18C6+ radical ions; the charge transfer occurs from H2O+ to B15C5 and B18C6 after the UV excitation. The position of the band origin for the H2O+·B18C6 complex (36323 cm–1) is almost the same as that for Rb+·B18C6 (36315 cm–1); the strength of the intermolecular interaction of H2O+ with B18C6 is similar to that of Rb+. The spectral features of the H2O+·B15C5 complex also resemble those of the Rb+·B15C5 ion. We measure IR–UV spectra of these complexes in the CH and OH stretching region. Four conformers are found for the H2O+·B15C5 complex, but there is one dominant form for the H2O+·B18C6 ion. This study demonstrates the production of radical ions by charge transfer from multivalent metal ions, their encapsulation by host molecules, and separate detection of their conformers by cold UV spectroscopy in the gas phase.

We measure UV photodissociation (UVPD) spectra of cold benzo-15-crown-5 (B15C5) and benzo-18-crown-6 (B18C6) complexes with divalent ions (M2+ = Ca2+, Sr2+, Ba2+, and Mn2+), solvated with an H2O or a CH3OH molecule: M2+·B15C5·H2O, M2+·B15C5·CH3OH, M2+·B18C6·H2O, and M2+·B18C6·CH3OH. All the species show a number of sharp vibronic bands in the 36600–37600 cm–1 region, which can be attributed to electronic transitions of the B18C6 or B15C5 component. Conformer-specific IR spectra of these complexes are also obtained by IR-UV double-resonance spectroscopy in the OH stretching region. All the IR-UV spectra of the H2O complexes show IR bands at ∼3610 and ∼3690 cm–1; these bands can be assigned to the symmetric and asymmetric OH stretching vibrations of the H2O component. The CH3OH complexes also show the stretching vibration of the OH group at ∼3630 cm–1. The H2O and the CH3OH components are directly bonded to the M2+ ion through the M2+···O bond in all the complexes, but a small difference in the conformation results in a noticeable difference in the OH stretching frequency, which enables us to determine the number of conformers. For Ca2+, Sr2+, and Mn2+, the number of conformers for the B18C6 complexes is in the range of 2–5, which is clearly larger than complexes with B15C5 (1 or 2). However, for Ba2+ the number of conformers with B18C6 (1 or 2) is almost the same as that with B15C5. This is probably because the Ba2+ ion is too large to be located in the cavity center of either B15C5 and B18C6, which provides an open site at the Ba2+ ion suitable for solvation with H2O or CH3OH. The more conformations a complex can take, the more entropically favored it is at nonzero temperatures. Hence, the larger number of conformations suggests higher stability of the complexes under solvated conditions, leading to a higher degree of ion encapsulation in solution.

We have measured electronic and conformer-specific vibrational spectra of hydrated dibenzo-18-crown-6 (DB18C6) complexes with potassium ion, K+•DB18C6•(H2O)n (n = 1–5), in a cold, 22-pole ion trap. We also present for comparison spectra of Rb+•DB18C6•(H2O)3 and Cs+•DB18C6•(H2O)3 complexes. We determine the number and the structure of conformers by analyzing the spectra with the aid of quantum chemical calculations. The K+•DB18C6•(H2O)1 complex has only one conformer under the conditions of our experiment. For K+•DB18C6•(H2O)n with n = 2 and 3, there are at least two conformers even under the cold conditions, whereas Rb+•DB18C6•(H2O)3 and Cs+•DB18C6•(H2O)3 each exhibit only one isomer. The difference can be explained by the optimum matching in size between the K+ ion and the crown cavity; because the K+ ion can be deeply encapsulated by DB18C6 and the interaction between the K+ ion and the H2O molecules becomes weak, different kinds of hydration geometries can occur for the K+•DB18C6 complex, giving multiple conformations in the experiment. For K+•DB18C6•(H2O)n (n = 4 and 5) complexes, only a single isomer is found. This is attributed to a cooperative effect of the H2O molecules on the hydration of K+•DB18C6; the H2O molecules form a ring, which is bound on top of the K+•DB18C6 complex. According to the stable structure determined in this study, the K+ ion in the K+•DB18C6•(H2O)n complexes tends to be pulled largely out from the crown cavity by the H2O molecules with increasing n. Multiple conformations observed for the K+ complexes will have an advantage for the effective capture of the K+ ion over the other alkali metal ions by DB18C6 because of entropic effects on the formation of hydrated complexes.

We report UV photodissociation (UVPD) and IR-UV double-resonance spectra of 1,2-dimethoxybenzene (DMB) complexes with alkali metal ions, M+·DMB (M = Li, Na, K, Rb, and Cs), in a cold, 22-pole ion trap. The UVPD spectrum of the Li+ complex shows a strong origin band. For the K+·DMB, Rb+·DMB, and Cs+·DMB complexes, the origin band is very weak and low-frequency progressions are much more extensive than that of the Li+ ion. In the case of the Na+·DMB complex, spectral features are similar to those of the K+, Rb+, and Cs+ complexes, but vibronic bands are not resolved. Geometry optimization with density functional theory indicates that the metal ions are bonded to the oxygen atoms in all the M+·DMB complexes. For the Li+ complex in the S0 state, the Li+ ion is located in the same plane as the benzene ring, while the Na+, K+, Rb+, and Cs+ ions are located off the plane. In the S1 state, the Li+ complex has a structure similar to that in the S0 state, providing the strong origin band in the UV spectrum. In contrast, the other complexes show a large structural change in the out-of-plane direction upon S1–S0 excitation, which results in the extensive low-frequency progressions in the UVPD spectra. For the Na+·DMB complex, fast charge transfer occurs from Na+ to DMB after the UV excitation, making the bandwidth of the UVPD spectrum much broader than that of the other complexes and producing the photofragment DMB+ ion.

Electronic and vibrational spectra of benzo-15-crown-5 (B15C5) and benzo-18-crown-6 (B18C6) complexes with alkali metal ions, M+•B15C5 and M+•B18C6 (M = Li, Na, K, Rb, and Cs), are measured using UV photodissociation (UVPD) and IR–UV double resonance spectroscopy in a cold, 22-pole ion trap. We determine the structure of conformers with the aid of density functional theory calculations. In the Na+•B15C5 and K+•B18C6 complexes, the crown ethers open the most and hold the metal ions at the center of the ether ring, demonstrating an optimum matching in size between the cavity of the crown ethers and the metal ions. For smaller ions, the crown ethers deform the ether ring to decrease the distance and increase the interaction between the metal ions and oxygen atoms; the metal ions are completely surrounded by the ether ring. In the case of larger ions, the metal ions are too large to enter the crown cavity and are positioned on it, leaving one of its sides open for further solvation. Thermochemistry data calculated on the basis of the stable conformers of the complexes suggest that the ion selectivity of crown ethers is controlled primarily by the enthalpy change for the complex formation in solution, which depends strongly on the complex structure.

We report UV photodissociation (UVPD) and IR-UV double-resonance spectra of dibenzo-18-crown-6 (DB18C6) complexes with alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) in a cold, 22-pole ion trap. All the complexes show a number of vibronically resolved UV bands in the 36 000–38 000 cm–1 region. The Li+ and Na+ complexes each exhibit two stable conformations in the cold ion trap (as verified by IR-UV double resonance), whereas the K+, Rb+, and Cs+ complexes exist in a single conformation. We analyze the structure of the conformers with the aid of density functional theory (DFT) calculations. In the Li+ and Na+ complexes, DB18C6 distorts the ether ring to fit the cavity size to the small diameter of Li+ and Na+. In the complexes with K+, Rb+, and Cs+, DB18C6 adopts a boat-type (C2v) open conformation. The K+ ion is captured in the cavity of the open conformer thanks to the optimum matching between the cavity size and the ion diameter. The Rb+ and Cs+ ions sit on top of the ether ring because they are too large to enter the cavity of the open conformer. According to time-dependent DFT calculations, complexes that are highly distorted to hold metal ions open the ether ring upon S1–S0 excitation, and this is confirmed by extensive low-frequency progressions in the UVPD spectra.

IR photodissociation (IRPD) spectra of [(N2O)nH2O]+ with n = 2−7 are measured in the 1100−3800 cm−1 region. The IRPD spectra show the ν1 and ν3 vibrations of the N2O components at around 1250 and 2200 cm−1 and the OH stretching vibrations of the H2O part in the 2400−3800 cm−1 region. In the OH stretching region, the IRPD spectrum of the [(N2O)2H2O]+ ion shows a sharp band at 3452 cm−1 and a broad one at ∼2700 cm−1, which are assignable to the stretching vibrations of the free and hydrogen-bonded OH groups, respectively. The IRPD spectrum of the [(N2O)3H2O]+ ion displays no band of the free OH stretching vibration; the solvent N2O molecules are preferentially hydrogen-bonded to the OH groups. In parallel, the geometry optimization and the vibrational analysis are carried out at the B3LYP/6-311++G(d,p) level of theory. Comparison of the IRPD spectra with the calculated IR spectra suggests that the [(N2O)nH2O]+ cluster ions have an (N2O·H2O)+ ion core, in which the positive charge is delocalized over the H2O and N2O components and that an intermolecular semicovalent bond is formed between the oxygen atoms of H2O and N2O through the charge resonance interaction. In the clusters larger than n = 3, two solvent N2O molecules are strongly hydrogen-bonded to the OH groups, and the other ones are weakly bound to the ion core. The band position of the ν1 vibration of the solvent N2O molecules suggests that the oxygen end of the solvent molecules is bonded to the ion core.

We report UV photodissociation (UVPD) and IR-UV double-resonance spectra of 1,2-dimethoxybenzene (DMB) complexes with alkali metal ions, M+·DMB (M = Li, Na, K, Rb, and Cs), in a cold, 22-pole ion trap. The UVPD spectrum of the Li+ complex shows a strong origin band. For the K+·DMB, Rb+·DMB, and Cs+·DMB complexes, the origin band is very weak and low-frequency progressions are much more extensive than that of the Li+ ion. In the case of the Na+·DMB complex, spectral features are similar to those of the K+, Rb+, and Cs+ complexes, but vibronic bands are not resolved. Geometry optimization with density functional theory indicates that the metal ions are bonded to the oxygen atoms in all the M+·DMB complexes. For the Li+ complex in the S0 state, the Li+ ion is located in the same plane as the benzene ring, while the Na+, K+, Rb+, and Cs+ ions are located off the plane. In the S1 state, the Li+ complex has a structure similar to that in the S0 state, providing the strong origin band in the UV spectrum. In contrast, the other complexes show a large structural change in the out-of-plane direction upon S1–S0 excitation, which results in the extensive low-frequency progressions in the UVPD spectra. For the Na+·DMB complex, fast charge transfer occurs from Na+ to DMB after the UV excitation, making the bandwidth of the UVPD spectrum much broader than that of the other complexes and producing the photofragment DMB+ ion.

The ultraviolet photodissociation (UVPD) spectra of calix[4]arene (C4A) complexes with alkali metal ions, M+·C4A (M = Na, K, Rb, and Cs), are measured in the 34000–37000 cm−1 region under cold (∼10 K) conditions in the gas phase. The UVPD spectra of the Na+·C4A and K+·C4A complexes show several sharp vibronic bands, while the UVPD spectra of the Rb+·C4A and Cs+·C4A complexes exhibit only broad features. The UVPD spectra are assigned with the aid of quantum chemical calculations. Most of the features in the UVPD spectra can be attributed to cone isomers, which are the most stable for all the M+·C4A complexes. In all the cone isomers, the M+ ion is encapsulated inside the cavity of C4A, and the structure is distorted to C2 symmetry from that of bare C4A (C4 symmetry). The cone isomers show a big difference in the electronic structure between the K+ and Rb+ complexes. The Rb+ and Cs+ complexes have an electronic structure similar to that of bare C4A. In the Na+ and K+ complexes, the two benzene rings facing each other in a pair have a short distance between them (<6 Å). This results in a substantial overlap of the π clouds between them, and an electronic transition is localized on this pair. Only this localized electronic transition of the Na+ and K+ complexes shows sharp band features in the UVPD spectra. In the Na+·C4A complex, the UVPD spectroscopic results suggest the coexistence of other isomers having partial-cone and 1,3-alternate forms. The energetics of the isomerization reactions of C4A and Na+·C4A is examined theoretically. The estimated potential barriers between the stable conformers are less than 75 kJ mol−1 for Na+·C4A, suggesting that conformational conversion can occur at room temperature, before the Na+·C4A complex enters the cold ion trap. The existence of multiple conformations for Na+·C4A is attributed to the higher stability of these conformers, both kinetically and thermodynamically, compared to the case of bare C4A and the other M+·C4A complexes.