Laser Spectroscopic Study of β‑Estradiol and Its Monohydrated Clusters in a Supersonic Jet

The structures of 17β-estradiol (estradiol) and its 1:1 cluster with water have been investigated in supersonic jets. The S₁–S₀ electronic spectrum of estradiol monomer shows four strong sharp bands in the 35050–35200 cm^–1 region. Ultraviolet–ultraviolet hole-burning (UV–UV HB) and infrared-ultraviolet double-resonance (IR-UV DR) spectra of these bands indicate that they are due to four different conformers of estradiol originating from the different orientation of the OH groups in the <i>A</i>- and <i>D</i>-rings. The addition of water vapor to the sample gas generates four new bands in the 34700–34800 cm^–1 region, which are assigned to the estradiol–H₂O 1:1 cluster with the <i>A</i>-ring (phenyl ring) OH acting as a hydrogen­(H)-bond donor. In addition, we found very weak bands near the origin bands of bare estradiol upon the addition of water vapor. These bands are assigned to the isomers of estradiol–H₂O 1:1 cluster having an H-bond at the <i>D</i>-ring OH. We determine the conformation of bare estradiol and the structures of its monohydrated clusters with the aid of density functional theory calculation and discuss the relationship between the stability of hydrated clusters and the conformation of estradiol

Structure and Hydrogen-Bonding Ability of Estrogens Studied in the Gas Phase

The structures of estrogens (estrone­(E1), β-estradiol­(E2), and estriol­(E3)) and their 1:1 hydrogen-bonded (hydrated) clusters with water formed in supersonic jets have been investigated by various laser spectroscopic methods and quantum chemical calculations. In the S₁–S₀ electronic spectra, all three species exhibit the band origin in the 35 050–35 200 cm^–1 region. By use of ultraviolet–ultraviolet hole-burning (UV–UV HB) spectroscopy, two conformers, four conformers, and eight conformers, arising from different orientation of OH group(s) in the A-ring and D-ring, are identified for estrone, β-estradiol, and estriol, respectively. The infrared–ultraviolet double-resonance (IR–UV DR) spectra in the OH stretching vibration are observed to discriminate different conformers of the D-ring OH for β-estradiol and estriol, and it is suggested that in estriol only the intramolecular hydrogen bonded conformer exists in the jet. For the 1:1 hydrated cluster of estrogens, the S₁–S₀ electronic transition energies are quite different depending on whether the water molecule is bound to A-ring OH or D-ring OH. It is found that the water molecule prefers to form an H-bond to the A-ring OH for estrone and β-estradiol due to the higher acidity of phenolic OH than that of the alcoholic OH. On the other hand, in estriol the water molecule prefers to be bound to the D-ring OH due to the formation of a stable ring-structure H-bonding network with two OH groups. Thus, the substitution of one hydroxyl group to the D-ring drastically changes the hydrogen-bonding preference of estrogens

UV and IR Spectroscopy of Cold H₂O⁺–Benzo-Crown Ether Complexes

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

Microhydration of Dibenzo-18-Crown‑6 Complexes with K⁺, Rb⁺, and Cs⁺ Investigated by Cold UV and IR Spectroscopy in the Gas Phase

In this Article, we examine the hydration structure of dibenzo-18-crown-6 (DB18C6) complexes with K⁺, Rb⁺, and Cs⁺ ion in the gas phase. We measure well-resolved UV photodissociation (UVPD) spectra of K⁺·DB18C6·(H₂O)_<i>n</i>, Rb⁺·DB18C6·(H₂O)_<i>n</i>, and Cs⁺·DB18C6·(H₂O)_<i>n</i> (<i>n</i> = 1–8) complexes in a cold, 22-pole ion trap. We also measure IR-UV double-resonance spectra of the Rb⁺·DB18C6·(H₂O)_1–5 and the Cs⁺·DB18C6·(H₂O)₃ complexes. The structure of the hydrated complexes is determined or tentatively proposed on the basis of the UV and IR spectra with the aid of quantum chemical calculations. Bare complexes (K⁺·DB18C6, Rb⁺·DB18C6, and Cs⁺·DB18C6) have a similar boat-type conformation, but the distance between the metal ions and the DB18C6 cavity increases with increasing ion size from K⁺ to Cs⁺. Although the structural difference of the bare complexes is small, it highly affects the manner in which each is hydrated. For the hydrated K⁺·DB18C6 complexes, water molecules bind on both sides (top and bottom) of the boat-type K⁺·DB18C6 conformer, while hydration occurs only on top of the Rb⁺·DB18C6 and Cs⁺·DB18C6 complexes. On the basis of our analysis of the hydration manner of the gas-phase complexes, we propose that, for Rb⁺·DB18C6 and Cs⁺·DB18C6 complexes in aqueous solution, water molecules will preferentially bind on top of the boat conformers because of the displaced position of the metal ions relative to DB18C6. In contrast, the K⁺·DB18C6 complex can accept H₂O molecules on both sides of the boat conformation. We also propose that the characteristic solvation manner of the K⁺·DB18C6 complex will contribute entropically to its high stability and thus to preferential capture of K⁺ ion by DB18C6 in solution

Anionic Polymerization Mechanism of Acrylonitrile Trimer Anions: Key Branching Point between Cyclization and Chain Propagation

A cluster anion of vinyl compounds in the gaseous phase has served as one of the simplest microscopic models of the initial stages of anionic polymerization. Herein, we describe our investigations into the initial stage mechanisms of anionic polymerization of acrylonitrile (AN; CH₂CHCN) trimer anions. While the cyclic oligomer is found in mass and photoelectron spectroscopic studies of (AN)₃^–, only the chain oligomer is found in the infrared photodissociation (IRPD) spectrum of Ar-tagged (AN)₃^–. On the basis of the calculated polymerization pathway of (AN)₃^–, we consider that the chain oligomers are the reaction intermediates in the cyclization of (AN)₃^–. The rotational isomerization of the (AN)₃^– chain oligomer is found to be the bottleneck in the cyclization of (AN)₃^–. To form the (AN)₄^– chain oligomer by chain propagation, the addition of an AN molecule to (AN)₃^– should occur prior to the rotational isomerization. We conclude that the rotational isomerization in the (AN)₃^– chain oligomer is the key branching point between cyclization (termination) or chain propagation in the anionic polymerization

Microhydration Effects on the Encapsulation of Potassium Ion by Dibenzo-18-Crown‑6

We have measured electronic and conformer-specific vibrational spectra of hydrated dibenzo-18-crown-6 (DB18C6) complexes with potassium ion, K⁺•DB18C6•(H₂O)_<i>n</i> (<i>n</i> = 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)_<i>n</i> with <i>n</i> = 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)_<i>n</i> (<i>n</i> = 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)_<i>n</i> complexes tends to be pulled largely out from the crown cavity by the H₂O molecules with increasing <i>n</i>. 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

Anomalous Cage Effect of the Excited State Dynamics of Catechol in the 18-Crown-6–Catechol Host–Guest Complex

We determined the number of isomers and their structures for the 18-crown-6 (18C6)–catechol host–guest complex, and examined the effect of the complex formation on the S₁ (¹ππ*) dynamics of catechol under a supersonically cooled gas phase condition and in cyclohexane solution at room temperature. In the gas phase experiment, UV–UV hole-burning spectra of the 18C6–catechol 1:1 complex indicate that there are three stable isomers. For bare catechol, it has been reported that two adjacent OH groups have an intramolecular hydrogen (H) bond. The IR–UV double resonance spectra show two types of isomers in the 18C6–catechol 1:1 complex; one of the three 18C6–catechol 1:1 isomers has the intramolecular H-bond between the two OH groups, while in the other two isomers the intramolecular H-bond is broken and the two OH groups are H-bonded to oxygen atoms of 18C6. The complex formation with 18C6 substantially elongates the S₁ lifetime from 7 ps for bare catechol and 2.0 ns for the catechol–H₂O complex to 10.3 ns for the 18C6–catechol 1:1 complex. Density functional theory calculations of the 18C6–catechol 1:1 complex suggest that this elongation is attributed to a larger energy gap between the S₁ (¹ππ*) and ¹πσ* states than that of bare catechol or the catechol–H₂O complex. In cyclohexane solution, the enhancement of the fluorescence intensity of catechol was found by adding 18C6, due to the formation of the 18C6–catechol complex in solution, and the complex has a longer S₁ lifetime than that of catechol monomer. From the concentration dependence of the fluorescence intensity, we estimated the equilibrium constant <i>K</i> for the 18C6 + catechol ⇄ 18C6–catechol reaction. The obtained value (log <i>K</i> = 2.3) in cyclohexane is comparable to those for alkali metal ions or other molecular ions, indicating that 18C6 efficiently captures catechol in solution. Therefore, 18C6 can be used as a sensitive sensor of catechol derivatives in solution with its high ability of fluorescence enhancement

Collision-Induced Fission of Oblate Gold Superatom in [Au₉(PPh₃)₈]³⁺: Deformation-Mediated Mechanism

Collision-induced dissociation (CID) patterns of the phosphine-protected Au-based clusters [PdAu8(PPh3)8]2+ (PdAu8) and [Au9(PPh3)8]3+ (Au9), featuring crown-shaped M@Au8 (M = Pd, Au) cores, were investigated. For PdAu8, ordinary sequential PPh3 losses (PdAu8 → [PdAu8(PPh3)m]2+ + (8 – m)PPh3 (m = 7, 6, 5)) were observed. In contrast, Au9 underwent cluster-core fission (Au9 → [Au6(PPh3)6]2+ (Au6) + [Au3(PPh3)2]+ (Au3)) upon sufficiently high energy collision, associated with splitting the number of valence electrons in the superatomic orbitals from 6e (Au9) into 4e (Au6) and 2e (Au3). Density functional theory calculations revealed oblate and prolate cores of Au9 and Au6 with semiclosed superatomic electron configurations of (1S)2(1Px)2(1Py)2 and (1S)2(1Pz)2, respectively. This result indicated a significant deformation of the cluster-core motif during the CID process. We attribute the clear difference between PdAu8 and Au9 to the softer Au–Au bond in Au9 and propose that the collision-induced structural deformation plays a critical role in the fission

Collision-Induced Fission of Oblate Gold Superatom in [Au₉(PPh₃)₈]³⁺: Deformation-Mediated Mechanism

Collision-induced dissociation (CID) patterns of the phosphine-protected Au-based clusters [PdAu8(PPh3)8]2+ (PdAu8) and [Au9(PPh3)8]3+ (Au9), featuring crown-shaped M@Au8 (M = Pd, Au) cores, were investigated. For PdAu8, ordinary sequential PPh3 losses (PdAu8 → [PdAu8(PPh3)m]2+ + (8 – m)PPh3 (m = 7, 6, 5)) were observed. In contrast, Au9 underwent cluster-core fission (Au9 → [Au6(PPh3)6]2+ (Au6) + [Au3(PPh3)2]+ (Au3)) upon sufficiently high energy collision, associated with splitting the number of valence electrons in the superatomic orbitals from 6e (Au9) into 4e (Au6) and 2e (Au3). Density functional theory calculations revealed oblate and prolate cores of Au9 and Au6 with semiclosed superatomic electron configurations of (1S)2(1Px)2(1Py)2 and (1S)2(1Pz)2, respectively. This result indicated a significant deformation of the cluster-core motif during the CID process. We attribute the clear difference between PdAu8 and Au9 to the softer Au–Au bond in Au9 and propose that the collision-induced structural deformation plays a critical role in the fission

Ultraviolet Photodissociation Spectroscopy of the Cold K⁺·Calix[4]arene Complex in the Gas Phase

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 (<i>C</i>₂ 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