IR-UV Double Resonance Spectroscopy Research Articles

Homochiral vs. heterochiral preference in chiral self-recognition of cyclic diols.

The structure and clustering propensity of a chiral derivative of cis-1,2-cyclohexanediol, namely, 1-phenyl-cis-1,2-cyclohexanediol (cis-PCD), has been studied under supersonic expansion conditions by combining laser spectroscopy with quantum chemistry calculations. The presence of the phenyl substituent induces conformational locking relative to cis-1,2-cyclohexanediol (cis-CD), and only one conformer of the bare molecule is observed by both Raman and IR-UV double resonance spectroscopy. The homochiral preference inferred for the dimer formation at low enough temperature is in line with the formation of a conglomerate in the solid state. The change in clustering propensity in cis-PCD relative to trans-1,2-cyclohexanediol (trans-CD), which shows heterochiral preference, is explained by the presence of the phenyl substituent rather than the effect of cis-trans isomerism. Indeed the transiently chiral cis-CD also forms preferentially heterodimers, whose structure is very close to that of the corresponding trans-CD dimer.

Far-IR Absorption of Neutral Polycyclic Aromatic Hydrocarbons (PAHs): Light on the Mechanism of IR-UV Ion Dip Spectroscopy.

Gas-phase IR–UV double-resonance laser spectroscopy is an IR absorption technique that bridges the gap between experimental IR spectroscopy and theory. The IR experiments are used to directly evaluate predicted frequencies and potential energy surfaces as well as to probe the structure of isolated molecules. However, a detailed understanding of the underlying mechanisms is, especially in the far-IR regime, still far from complete, even though this is crucial for properly interpreting the recorded IR absorption spectra. Here, events occurring upon excitation to vibrational levels of polycyclic aromatic hydrocarbons by far-IR radiation from the FELIX free electron laser are followed using resonance-enhanced multiphoton ionization spectroscopy. These studies provide detailed insight into how ladder climbing and anharmonicity influence IR–UV spectroscopy and therefore the resulting IR signatures in the far-IR region. Moreover, the potential energy surfaces of these low-frequency delocalized modes are investigated and shown to have a strong harmonic character.

Observation of a weak intra-residue C5 hydrogen-bond in a dipeptide containing Gly-Pro sequence.

Specific folded structures of peptides and proteins depend on the sequence of various amino acid residues as well as different types of noncovalent interactions induced by the backbone as well as side-chains of those residues. In general, secondary structures of peptides and proteins are stabilized by C6 (δ-turn), C7 (γ-turn), C10 (β-turn), C13 (α-turn), and C15 (π-turn) hydrogen-bonded rings formed through inter-residue interactions. However, it has been reported recently that an intraresidue C5 hydrogen-bond, which is relatively weak in strength, can contribute significantly to the stability of peptides and proteins. The C5 hydrogen-bond is mostly present in the β-sheet structures of peptides and proteins along with other inter-residue noncovalent interactions. In this work, we have studied structures and conformational preferences of a dipeptide Z-Gly-Pro-OH (Z = benzyloxycarbonyl) using mass-selected vibrationally resolved electronic spectroscopy and IR-UV double resonance spectroscopy coupled with quantum chemistry calculations. Two conformers of the peptide are observed in the experiment. One of the conformers has an extended β-strand type structure stabilized by C5 hydrogen-bonding, while the other one is folded through O-H ⋯ π interaction. The noncovalent interactions present in the two observed structures of the peptide are validated by natural bond orbital and noncovalent interaction calculations.

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

Ultraviolet photodissociation (UVPD) and IR-UV double-resonance spectroscopy are performed for bare and microhydrated complexes of Mn2+(benzo-15-crown-5), Mn2+(B15C5)(H2O)n (n = 0-2), under cold (∼10 K) 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. Time-dependent DFT calculations suggest that the π-π* transition of the B15C5 part is highly mixed with the "ligand to metal charge transfer" 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.

Conformation of alkali metal ion-calix[4]arene complexes investigated by IR spectroscopy in the gas phase.

We measure the IR spectra of calix[4]arene (C4A) complexes with K+, Rb+, and Cs+ ions in the 3200-3700 cm-1 region by IR-UV double-resonance spectroscopy performed under cold (∼10 K) gas-phase conditions. All the complexes show two bands that can be assigned to the stretching vibrations of hydrogen-bonded OH groups in the C4A part. Quantum chemical calculations predict several isomers having different IR spectra, but the IR spectrum of the "cone" conformer reproduces the IR-UV spectrum very well, indicating that all the complexes adopt the cone conformation including the metal ions in the cone. The frequency of the OH stretching vibrations decreases with increasing the ion size from K+ (3357 and 3513 cm-1) to Rb+ (3323 and 3463 cm-1) and Cs+ (3279 and 3379 cm-1), but it is substantially higher than that of hydrogen-bonded OH groups in bare C4A (3158 cm-1). These results suggest that C4A encapsulates the metal ions by distorting the cone cavity, and that the distortion of the cone conformation is reduced more and the hydrogen bond between the OH groups becomes stronger with increasing the ion size from K+ to Cs+. The Cs+ complex has the smallest distortion of the C4A cavity among the alkali metal ion complexes. This can be one origin for the predominant encapsulation of Cs+ ions by C4A over smaller alkali metal ions in solution.

Laser spectroscopic study on sinapic acid and its hydrated complex in a cold gas phase molecular beam

The S1 (ππ∗)-S0 electronic spectrum and the infrared spectrum in the OH stretching region of sinapic acid (SA) and its 1:1 complex with water (SA-H2O) are observed in a supersonically cooled molecular beam. Supersonically-cooled SA and SA-H2O complex are obtained by a combination of laser ablation with a newly developed pulsed channel nozzle. Both SA and SA-H2O exhibit sharp electronic spectra, and measurements of UV-UV hole-burning spectra and time-dependent density functional theory (TD-DFT) calculation indicate that the observed vibronic bands belong to two conformers, syn and anti, in both SA and the SA-H2O complex. The OH stretching vibrations are observed with IR-UV double resonance spectroscopy. A comparison of the observed IR spectrum and the DFT-calculated one indicates that SA-H2O has the structure in which a carboxylic (COOH) group and the water form a cyclic hydrogen (H)-bond. The OH stretch of COOH is red-shifted as large as 600 cm−1 in the complex. The S1 lifetime of SA at the zero-point energy level is obtained to be 1.9 ns. This S1 lifetime is shorter than the reported one of its ester derivative (methyl sinapate, MS (Dean et al., J. Am. Chem. Soc. 136 (2014) 1478–14795)) but is much longer than the para-substituted cinnamates, p-hydroxy methylcinnamate (p-HMC (Shimada et al., Phys. Chem. Chem. Phys. 14 (2012) 8999–9005)) and p-methoxy methylcinnamate (p-MMC (Miyazaki et al., J. Chem. Phys. 141 (2014) 244313)). A calculation at TD-DFT level indicates that this long lifetime of SA is due to that the 1nπ∗ state and the transition states along the trans → cis isomerization in S1 (ππ∗) are located much higher in energy than p-HMC and p-MMC. This may lead a main nonradiative channel of SA in the low energy region to be the multistep intersystem crossing to T1 (3ππ∗) via the 3nπ∗ state.

Hydrogen bond induced enhancement of Fermi resonances in N-HN hydrogen bonded complexes of anilines.

The hydrogen-bonded complexes of aniline, 4-fluoroaniline and 4-ethynylaniline with ammonia, methylamine, dimethylamine, trimethylamine and triethylamine, were investigated using IR-UV double resonance spectroscopy. The formation of N-HN hydrogen bonded complexes with anilines as donors and alkylamines as acceptors was inferred from the appearance of the spectra. Two bands appearing in the 3100-3400 cm-1 region were found to be originating from the Fermi resonance coupling between the hydrogen-bonded NH2 stretching and NH2 bend-overtone vibrations. A two-state de-perturbation analysis yields the zero-order (unperturbed) vibrational states and the coupling constant. An inverse correlation between the zero-order hydrogen-bonded NH2 stretching and NH2 bend-overtone was observed due to a switch in the relative contributions of hydrogen-bonded NH2 stretching and NH2 bend-overtone vibrations to the Fermi resonance bands. These results lead to the reassignment of the hydrogen-bonded N-H stretching frequencies of aniline complexes reported earlier. Furthermore, the stretch-bend Fermi-resonance coupling constant for the NH2 group is around 50 cm-1, which is independent of the nature of the parent donor molecule and the acceptor, and is intrinsic to the NH2 group.

Stereochemistry-dependent structure of hydrogen-bonded protonated dimers: the case of 1-amino-2-indanol.

To understand the role of chirality in shaping biological supramolecular systems it is instructive to visualize the subtle effects of stereochemistry on the structure of model aggregates at the molecular level. Here, we apply conformer-specific IR-UV double-resonance laser spectroscopy in a cold ion trap to derive a detailed description of the protonated homodimers of (1R,2S)-cis- and (1R,2R)-trans-1-amino-2-indanol (c-AI2H+, t-AI2H+). Although the protonated monomers (c-AIH+, t-AIH+) only differ by the chirality of one carbon atom, their conformations are clearly distinct. c-AIH+ has an intramolecular NH+O hydrogen bond (H-bond), while t-AIH+ lacks such an interaction. This has crucial consequences on the geometry and stability of the corresponding c-AI2H+ and t-AI2H+ dimers. While there is a competition between intra- and intermolecular H-bonds in c-AI2H+, the formation of t-AI2H+ does not require deformation of the monomers. This difference results in higher binding energies of t-AI2H+ compared to c-AI2H+. To optimize the H-bond network, the two dimers do not necessarily involve the corresponding most stable monomers. c-AI2H+ and t-AI2H+ differ in their UV photodissociation mass spectra and in their electronic spectra, which suggests different geometries also in the excited state.

Intrinsic structure of pentapeptide Leu-enkephalin: geometry optimization and validation by comparison of VSCF-PT2 calculations with cold ion spectroscopy.

The intrinsic structure of an opioid peptide [Ala2, Leu5]-leucine enkephalin (ALE) has been investigated using first-principles based vibrational self-consistent field (VSCF) theory and cold ion spectroscopy. IR-UV double resonance spectroscopy revealed the presence of only one highly abundant conformer of the singly protonated ALE, isolated and cryogenically cooled in the gas phase. High-level quantum mechanical calculations of electronic structures in conjunction with a systematic conformational search allowed for finding a few low-energy candidate structures. In order to identify the observed structure, we computed vibrational spectra of the candidate structures and employed the theory at the semi-empirically scaled harmonic level and at the first-principles based anharmonic VSCF levels. The best match between the calculated "anharmonic" and the measured spectra appeared, indeed, for the most stable candidate. An average of two spectra calculated with different quantum mechanical potentials is proposed for the best match with experiment. The match thus validates the calculated intrinsic structure of ALE and demonstrates the predictive power of first-principles theory for solving structures of such large molecules.

Conformational Heterogeneity and the Role of the C(2)-H Donor in Mono- and Dihydrated Clusters of Benzoxazole.

The significance of the heteroatom in influencing the acidity and binding affinity of the C(2)-H donor in five-membered heterocyclic rings is explored. The water clusters of benzoxazole (BOX) are studied in a supersonic jet by IR-UV double resonance spectroscopy and compared with those of benzimidazole (BIM) and its N-methyl derivative (MBIM). Two conformers are identified for the monohydrated cluster, both of which are O-H···N bound and differ in their orientation with respect to the intermolecular hydrogen bond. IR spectroscopy of the doubly hydrated cluster shows the presence of an intensity enhanced C(2)-H (carbon atom between the heteroatoms in the five-membered ring) stretching mode in addition to two red-shifted bound OH stretches, indicating that the water molecules form a hydrogen-bonded bridge encompassing the N acceptor and the weakly activated C(2)-H bond in oxazole. Comparison of the topological parameters of electron density, natural bond orbital analyses, and computed binding energies of the analogous hydrated structures of BOX, BIM, and MBIM indicates that the C(2)-H bond in the former is a more potent H-bond donor.

Excited State Dynamics of 6-Thioguanine.

Here we present the excited state dynamics of jet-cooled 6-thioguanine (6-TG), using resonance-enhanced multiphoton ionization (REMPI), IR-UV double resonance spectroscopy, and pump-probe spectroscopy in the nanosecond and picosecond time domains. We report data on two thiol tautomers, which appear to have different excited state dynamics. These decay to a dark state, possibly a triplet state, with rates depending on tautomer form and on excitation wavelength, with the fastest rate on the order of 1010 s-1. We also compare 6-TG with 9-enolguanine, for which we observed decay to a dark state with a 2 orders of magnitude smaller rate. At increased excitation energy (∼+500 cm-1) an additional pathway appears for the predominant thiol tautomer. Moreover, the excited state dynamics for 6-TG thiols is different from that recently predicted for thiones.

Interplay among Electrostatic, Dispersion, and Steric Interactions: Spectroscopy and Quantum Chemical Calculations of π-Hydrogen Bonded Complexes.

π-Hydrogen bonding interactions are ubiquitous in both materials and biology. Despite their relatively weak nature, great progress has been made in their investigation by experimental and theoretical methods, but this becomes significantly more complicated when secondary intermolecular interactions are present. In this study, the effect of successive methyl substitution on the supramolecular structure and interaction energy of indole⋅⋅⋅methylated benzene (ind⋅⋅⋅n-mb, n=1-6) complexes is probed through a combination of supersonic jet experiments and benchmark-quality quantum chemical calculations. It is demonstrated that additional secondary interactions introduce a subtle interplay among electrostatic and dispersion forces, as well as steric repulsion, which fine-tunes the overall structural motif. Resonant two-photon ionization and IR-UV double-resonance spectroscopy techniques are used to probe jet-cooled ind⋅⋅⋅n-mb (n=2, 3, 6) complexes, with redshifting of the N-H IR stretching frequency showing that increasing the degree of methyl substitution increases the strength of the primary N-H⋅⋅⋅π interaction. Ab initio harmonic frequency and binding energy calculations confirm this trend for all six complexes. Electronic spectra of the three dimers are broad and structureless, with quantum chemical calculations revealing that this is likely to be due to multiple tilted conformations of each dimer possessing similar stabilization energies.

UV and IR Spectroscopy of Cryogenically Cooled, Lanthanide-Containing Ions in the Gas Phase.

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.

Laser Spectroscopic Study of Cold Gas-Phase Host-Guest Complexes of Crown Ethers.

The structure, molecular recognition, and inclusion effect on the photophysics of guest species are investigated for neutral and ionic cold host-guest complexes of crown ethers (CEs) in the gas phase. Here, the cold neutral host-guest complexes are produced by a supersonic expansion technique and the cold ionic complexes are generated by the combination of electrospray ionization (ESI) and a cryogenically cooled ion trap. The host species are 3n-crown-n (3nCn; n = 4, 5, 6, 8) and (di)benzo-3n-crown-n ((D)B3nCn; n = 4, 5, 6, 8). For neutral guests, we have chosen water and aromatic molecules, such as phenol and benzenediols, and as ionic species we have chosen alkali-metal ions (M(+) ). The electronic spectra and isomer-specific vibrational spectra for the complexes are observed with various laser spectroscopic methods: laser-induced fluorescence (LIF); ultraviolet-ultraviolet hole-burning (UV-UV HB); and IR-UV double resonance (IR-UV DR) spectroscopy. The obtained spectra are analyzed with the aid of quantum chemical calculations. We will discuss how the host and guest species change their flexible structures for forming best-fit stable complexes (induced fitting) and what kinds of interactions are operating for the stabilization of the complexes. For the alkali metal ion•CE complexes, we investigate the solvation effect by attaching water molecules. In addition to the ground-state stabilization problem, we will show that the complexation leads to a drastic effect on the excited-state electronic structure and dynamics of the guest species, which we call a "cage-like effect".

Solvent Effects on the Encapsulation of Divalent Ions by Benzo-18-Crown-6 and Benzo-15-Crown-5.

We measure UV photodissociation (UVPD) spectra of cold benzo-15-crown-5 (B15C5) and benzo-18-crown-6 (B18C6) complexes with divalent ions (M(2+) = Ca(2+), Sr(2+), Ba(2+), and Mn(2+)), solvated with an H2O or a CH3OH molecule: M(2+)·B15C5·H2O, M(2+)·B15C5·CH3OH, M(2+)·B18C6·H2O, and M(2+)·B18C6·CH3OH. All the species show a number of sharp vibronic bands in the 36,600-37,600 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 M(2+) ion through the M(2+)···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 Ca(2+), Sr(2+), and Mn(2+), 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 Ba(2+) the number of conformers with B18C6 (1 or 2) is almost the same as that with B15C5. This is probably because the Ba(2+) ion is too large to be located in the cavity center of either B15C5 and B18C6, which provides an open site at the Ba(2+) 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.

UV photofragmentation and IR spectroscopy of cold, G-type β-O-4 and β-β dilignol-alkali metal complexes: structure and linkage-dependent photofragmentation.

Ultraviolet photofragmentation spectroscopy and infrared spectroscopy were performed on two prototypical guaiacyl (G)-type dilignols containing β-O-4 and β-β linkages, complexed with either lithium or sodium cations. The complexes were generated by nanoelectrospray ionization, introduced into a multistage mass spectrometer, and subsequently cooled in a 22-pole cold ion trap to T ≈ 10 K. A combination of UV photofragment spectroscopy and IR-UV double resonance spectroscopy was used to characterize the preferred mode of binding of the alkali metal cations and the structural changes so induced. Based on a combination of spectral evidence provided by the UV and IR spectra, the Li(+) and Na(+) cations are deduced to preferably bind to both dilignols via their linkages, which constitute unique, oxygen-rich binding pockets for the cations. The UV spectra reflect this binding motif in their extensive Franck-Condon activity involving low-frequency puckering motions of the linkages in response to electronic excitation. In the pinoresinol•Li(+)/Na(+) complexes involving the β-β linkage, the spectra also showed an inherent spectral broadening. The photofragment mass spectra are unique for each dilignol•Li(+)/Na(+) complex, many of which are also complementary to those produced by collision-induced dissociation (CID), indicating the presence of unique excited state processes that direct the fragmentation. These results suggest the potential for site-selective fragmentation and for uncovering fragmentation pathways only accessed by resonant UV excitation of cold lignin ions.

Assessment of amide I spectroscopic maps for a gas-phase peptide using IR-UV double-resonance spectroscopy and density functional theory calculations.

The spectroscopy of amide I vibrations has become a powerful tool for exploring protein structure and dynamics. To help with spectral interpretation, it is often useful to perform molecular dynamics (MD) simulations. To connect spectroscopic experiments to simulations in an efficient manner, several researchers have proposed "maps," which relate observables in classical MD simulations to quantum spectroscopic variables. It can be difficult to discern whether errors in the theoretical results (compared to experiment) arise from inaccuracies in the MD trajectories or in the maps themselves. In this work, we evaluate spectroscopic maps independently from MD simulations by comparing experimental and theoretical spectra for a single conformation of the α-helical model peptide Ac-Phe-(Ala)5-Lys-H(+) in the gas phase. Conformation-specific experimental spectra are obtained for the unlabeled peptide and for several singly and doubly (13)C-labeled variants using infrared-ultraviolet double-resonance spectroscopy, and these spectra are found to be well-modeled by density functional theory (DFT) calculations at the B3LYP/6-31G** level. We then compare DFT results for the deuterated and (13)C(18)O-labeled peptide with those from spectroscopic maps developed and used previously by the Skinner group. We find that the maps are typically accurate to within a few cm(-1) for both frequencies and couplings, having larger errors only for the frequencies of terminal amides.

Observation of exclusively π-stacked heterodimer of indole and hexafluorobenzene in the gas phase

In this study, the structure of the indole···hexafluorobenzene dimer has been investigated in the gas phase by using resonant two photon ionzation (R2PI) and IR-UV double resonance spectroscopy combined with quantum chemistry calculations. We have confirmed the presence of exclusively π-stacked structure of the dimer from both experimental and theoretical IR spectra in the N-H stretching region. Observation of a single stable structure of the dimer has also been verified through 3D potential energy surface scan of the π-stacked dimer by varying the parallel displacement of the hexafluorobenzene unit simultaneously along the major and minor axes of the indole moiety. π-stacking interaction is present very often between the tryptophan and phenylalanine residues in proteins. But this interaction has not been observed earlier in the gas phase experiment by studying indole···benzene dimer because the N-H group of indole predominately directs towards the N-H···π hydrogen bonded T-shaped structure. The chosen molecular systems in this study not only rule out the possibility of the formation of the N-H···π bound T-shaped dimer but also enable the determination of the structure by probing the N-H group. The π-stacked indole···hexafluorobenzene dimer has a unique structure where the center of the hexafluorobenznene ring is aligned with the center of the shared bond of the indole ring. Our work provides useful insight in designing unnatural proteins having strong π-stacking interaction between the tryptophan and phenylalanine residues.

IR–UV Double Resonance Spectroscopy as Implemented by Polarized Laser Schemes: Probing Orientations of Vibrational Transition Dipole Moments

Abstract IR–UV double resonance spectroscopy based on polarized laser schemes is developed in order to gain information on the orientations of vibrational transition dipole moments. Polarization dependence of IR band shape is presented for the two amino stretching modes of aniline.

A novel ion trap based tandem mass spectrometer for the spectroscopic study of cold gas phase polyatomic ions

A novel tandem mass spectrometry based instrument for the spectroscopic interrogation of cold gas phase polyatomic ions has been constructed. The instrument consists of a dual linear ion trap (LIT) based triple quadrupole axis intersecting a spectroscopic axis containing a 22-pole ion trap cooled to 5K. The triple quadrupole axis intersects the spectroscopy axis between the second and third quadrupoles, which are separated by an ion deflector that is used to direct ion injection into the cold ion trap from the second quadrupole and subsequently to direct ions ejected from the cold ion trap into the third quadrupole. Both the second and third quadrupoles can be operated as LITs capable of dipolar excitation across opposing quadrupole rods. Broad-band or single-frequency waveforms can be used to effect mass selection or mass analysis, respectively. The dual ion trapping capability allows for ion accumulation to occur in parallel with ion spectroscopy and mass analysis, thereby improving the overall efficiency of the experiment. Extensive use of homebuilt equipment has allowed for maximum flexibility, with capabilities for using ion/ion reactions in the ion generation step, and IR-UV double resonance spectroscopy during ion interrogation.