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Slow Photoelectron Spectroscopy of 3‑Hydroxyisoquinoline Yi Pan and Kai-Chung Lau Department of Biology and Chemistry, City University of Hong Kong, Kowloon, Hong Kong

Lionel Poisson CNRS, IRAMIS, SPAM, Laboratoire Francis Perrin, URA 2453, F-91191 Gif-sur-Yvette, France. CEA, IRAMIS, SPAM, Laboratoire Francis Perrin, URA 2453, F-91191 Gif-sur-Yvette, France.

Gustavo A. Garcia and Laurent Nahon Synchrotron SOLEIL, L’orme des Merisiers, Saint-Aubin - BP 48 - 91192 Gif-sur-Yvette Cedex, France.

Majdi Hochlaf* Laboratoire Modélisation et Simulation Multi Echelle, Université Paris-Est, MSME UMR 8208 CNRS, 5 bd Descartes, 77454 Marne-la-Vallée, France S Supporting Information *

ABSTRACT: We studied the single photon ionization of gas phase 3-hydroxyisoquinoline by means of VUV synchrotron radiation coupled to a velocity map imaging electron/ion coincidence spectrometer. Near the ionization thresholds of 3hydroxyisoquinoline, the photoionization is found to occur mainly via a direct process. The spectra are assigned with the help of theoretical calculations on the equilibrium geometries, electronic states patterns, harmonic and anharmonic wavenumbers of the lactim and lactam forms of 3-hydroxyisoquinoline and their cations. The slow photoelectron spectrum (SPES) of this lactim is dominated by vibrational transitions to the X̃ state of the cation. In addition, several weaker and complex bands are observed, corresponding to the population of the vibrational bands (pure or combination) of the Ã electronically excited state of the cation. The adiabatic ionization energy of 3hydroxyisoquinoline and the lowest electronic state energetics of the lactim and lactam cationic forms are determined. of 2-pyridone,5 the 3-hydroxyisoquinoline is dominated by the lactim (enol) form. There are a few experimental and theoretical studies on 3hydroxyisoquinoline. In 1998, Wei et al. studied the proton transfer tautomerism of 3-hydroxyisoquinoline and its derivatives at the HF/6-31G(d) level of theory.6 Their combined experimental and theoretical results showed that the conjugated dual hydrogen-bonding effect plays an important role in both ground and excited states of 3-hydroxyisoquinoline. The mechanisms of tautomerization on the first excited state of 3hydroxyisoquinoline as well as in 1:1 mixtures of water and acetic acid were theoretically studied at CIS/6-31G(d) level by Ramos et al. in 2000.7 An energy barrier of 51.1 kcal·mol−1 was

I. INTRODUCTION In the past decades, 3-hydroxyisoquinoline (Figure 1) and its derivatives attracted interests from both chemists and biologists. The 3-hydroxyisoquinoline is considered as a prototype for the lactam−lactim tautomerism of nitrogencontaining heteroaromatic systems since the 1960s.1,2 The 3hydroxyisoquinoline derivatives are important reactants and intermediates in the synthesis of some chemical catalysts such as 1-(2′-diphenylphosphino-1′-naphthyl)-isoquinoline (QUINAP), an axially chiral ligand used in asymmetric alkene hydroboration.3 Recently, a study indicated that 3-hydroxyisoquinolines can be used as inhibitors of Hepatitis C Virus (HCV) ribonucleic acid (RNA) polymerase.4 Similar to 2pyridone, 3-hydroxyisoquinoline is a DNA base analogue and can be used as a prototype for studying the effect of electron delocalization on the aromatic ring as well as the electronic structure of DNA bases. Unlike the two isoenergetic tautomers © 2013 American Chemical Society

Special Issue: Stereodynamics Symposium Received: November 26, 2012 Published: January 29, 2013 8095

dx.doi.org/10.1021/jp311615u | J. Phys. Chem. A 2013, 117, 8095−8102

The Journal of Physical Chemistry A

Article

CABS(OptRI) basis sets of Peterson.24 In addition, the core− valence (CV) and scalar−relativistic (SR) corrections were included. The CV effects are the difference between electronic energies with only valence electrons correlated and that with both core and valence electrons correlated at the (R)CCSD(T)/cc-pwCVTZ level of theory.25,26 The core electrons are the 1s electrons of carbon, nitrogen, and oxygen. The SR energetic contributions are taken as the difference between electronic energies at the (R)CCSD(T)/cc-pVTZ level27 without using the spin-free, one-electron Douglas−Kroll−Hess (DKH) Hamiltonian28,29 and at the (R)CCSD(T)/cc-pVTZ-DK level30 with the DKH Hamiltonian. For electronic state computations, the complete active-space self-consistent field (CASSCF)31,32 and the internally contracted multireference configuration interaction (MRCI) approaches33,34 were used together with the aug-cc-pVDZ basis set.35 In the CASSCF calculations, the electronic states were averaged with equal weights. In the Cs point group, the molecular orbitals (MOs) of the 3-hydroxyisoquinoline consist of 11 a′ core orbitals and 40 a′ + 11 a″ valence orbitals. Due to the computational hardware limitation, several tests were performed to determine the appropriate size of active space without significant change in the order of electronic states of the 3-hydroxyisoquinoline in the 0−6 eV internal energy domain. Presently, the best active space, as a compromise between computational time and accuracy, comprises a set of 26 a′ + 8 a″ valence orbitals (active) and 11 a′ core orbitals (frozen). Out of the 26 a′ + 8 a″ valence orbitals, the first 18 a′ + 3 a″ valence orbitals were kept as closed orbitals during the CASSCF calculations. In the doublet cations, ∼4.9 × 105 configuration state functions (CSFs) were treated. At the MRCI level, all configurations with coefficients larger than 0.5 in the CI expansion of the CASSCF wave functions were used as a reference, and seven more a′ valence orbitals were frozen. This leads to ∼1.2 × 108 uncontracted configurations to be treated in the cations. The (R)CCSD(T)-F12, CASSCF, and MRCI methods are all implemented in the MOLPRO (version 2010.1) program suite.36

Figure 1. Optimized structure of singlet 3-hydroxyisoquinoline (X̃ 1A′) obtained at the PBE0/6-311+G(d,p) level. The numbering of the atoms used in Table 1 is also given.

predicted for tautomerization of the isolated 3-hydroxyisoquinoline molecule in the ground state, while the barrier was lowered by ∼30 and ∼40 kcal·mol−1 in the 1:1 mixture with water and acetic acid, respectively. In 2007, Gerega et al.8 investigated the stabilities of 3-hydroxyisoquinoline lactim and lactam forms experimentally and theoretically. The lactam form was not observed in the experimental IR spectra in Ar matrices, but it was theoretically found to be ∼30 kJ·mol−1 higher in energy than the lactim at the QCISD/cc-pVDZ and QCISD(T)/cc-pVDZ levels based on the geometries at the B3LYP/ccpVDZ level. To our knowledge, there is no prior study on 3hydroxyisoquinoline cations (lactim+ and lactam+). Our experimental and theoretical works will focus on the spectroscopy of the cationic form of 3-hydroxyisoquinoline lactim. The ions are produced from single-photon photoionization of the neutral molecule in a jet-cooled molecular beam. The slow photoelectron spectra (SPES)9−11 present rich structures corresponding to the population of vibrational levels of the lactim+. The interpretation and assignment of the experimental spectrum are based on the theoretical calculations on the structures, vibrational harmonic and anharmonic frequencies, and the electronic excitation energies of the lactim+ and lactam+.

III. EXPERIMENTAL METHODOLOGIES The experimental procedure was described in detail in refs 9−11. Briefly, the monochromatized VUV ionizing radiation is delivered by the DESIRS beamline,37 at the third generation, French synchrotron facility SOLEIL located in St. Aubin, France. The undulator radiation harmonics other than the fundamental are absorbed by a gas filter standing upstream of the beamline,38 which was filled with 0.17 mbars of Kr to avoid fast electron background and fragmentation due to high-energy photons that could be transmitted by the high orders of the monochromator grating. For this experiment, we used a moderate photon resolution giving a typical bandwidth of about 2.5 meV and an absolute energy calibration precision of ±3 meV. The photon flux was monitored and normalized owing to a VUV Photodiode (IRD AXUV100). The photoionization region is defined by the crossing of the VUV light direction, the molecular jet and the spectrometer (at 90° of each other). For these experiments, we used the DELECIOUS II spectrometer,39 where the electrons and ions are extracted in opposite directions toward a velocity map imaging (VMI)40 and a Wiley−McLaren time-of-flight (TOF) spectrometer, respectively, for signal collection in coincidence. Thus, photoelectron images can be obtained for a particular mass. The spectrometer is capable of recording electron kinetic

II. COMPUTATIONAL DETAILS The optimizations of molecular structures, as well as the harmonic and anharmonic frequencies calculations, were performed at the PBE0/6-311+G(d,p) level12 (using the ultrafine integral grids) in the GAUSSIAN09 program.13 All calculations were carried out in the Cs point group. The 6311+G(d,p) basis set results in 284 contracted Gaussian functions (GTOs) for the 3-hydroxyisoquinoline and its cations. For better accuracy, explicitly correlated computations14−16 at the (R)CCSD(T)-F12/cc-pVTZ-F12 (approximation B) level are performed on the tautomerization between lactam and lactim as well as their cations based on the PBE0 optimized structures. For these large molecular systems, such calculations are feasible since CPU and disk space are reduced by up to 2 orders of magnitude when using (R)CCSD(T)-F12 instead of (R)CCSD(T) for similar accuracy.17−19 Within the explicitly correlated computations, the C, H, N, and O atoms were described using the cc-pVTZ-F12 explicitly correlated basis sets,20 in connection with the corresponding auxiliary basis sets and density fitting functions.21−23 Indeed, we used the default 8096



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energies ranging from 0 up to 17 eV with a maximum resolution of 4.5% and a 4π collection efficiency. It offers an ion mass resolution of ∼130 amu. In the present study, the extraction field was set to 18 V/cm, a value optimized to give threshold electrons with a resolution down to ∼9 meV. With this extraction field, a 100% collection efficiency for electrons possessing kinetic energies less than 190 meV is ensured. Note also that the DC extraction field will induce a small red-shift of 3 meV in our IEs that we have corrected accordingly. The jet conditions (carrier gas, backing pressure, oven temperature) were roughly optimized to mainly produce the monomer of 3-hydroxyisoquinoline. A commercial sample of this compound (Purchased from Sigma-Aldrich with 97% purity) was placed in an in-vacuum temperature-controlled oven installed inside the SAPHIRS molecular beam chamber. The oven was heated to 180 °C, and the resulting vapor mixed with 0.5 bar of He and expanded through a 50 μm nozzle into a first (expansion) chamber maintained at 1.2 × 10−5 mbar. The supersonic beam then enters a second, ionization chamber through a 1 mm skimmer where it crosses the photon beam at a right angle in the center of the DELICIOUS II spectrometer. At each photon energy, the mass-selected photoelectron VMI images are Abel-transformed to recover the original radial (kinetic energy) distributions.9,41 The photoelectron spectra can then be plotted as a function of the photon energy in 2D matrix form. Since for direct photoionization, the energy of the photoelectrons increases with the photon energy, these 2D spectra consist of a set of constant slope, parallel lines corresponding to the different cationic levels. Note that the polarization vector of the radiation was orthogonal to the VMI detector, so that an anisotropy parameter (β) equal to zero has to be assumed in order to treat the VMI images.

Table 1. Main Geometrical Parameters (Å and deg) of the Neutral and Cationic 3-Hydroxyisoquinoline, Obtained at the PBE0/6-311+G(d,p) Level of Theory bonds and angles N16−C14 C14−O17 C14−C10 C10−C3 C3−C2 C2−C1 C1−C6 C6−C5 C5−C4 C4−C3 C4−C11 C11−N16 O17−H18 N16−H18 C10−H7 C2−H9 C1−H8 C6−H13 C5−H12 C11−H15 N16−C14−O17 C10−C14−O17 C14−O17−H18 N16−C14−C10 C14−C10−C3 C10−C3−C4 C3−C4−C11 C4−C11−N16 C11−N16−C14 C4−C3−C2 C3−C2−C1 C2−C1−C6 C1−C6−C5 C6−C5−C4 C5−C4−C3

IV. RESULTS AND DISCUSSION a. Theoretical Results. The equilibrium geometrical parameters of 3-hydroxyisoquinoline and its cations, including the lactim and lactam forms, at the PBE0/6-311+G(d,p) level are given in Table 1. Inspecting the geometrical parameters reveals that the main changes between the neutral lactim and its cation (lactim+) are the bond lengths of C14−O17 (∼0.04 Å) and C14−C10 (∼0.05 Å) and the bond angles of N16−C14−O17 (∼2°), C10−C14−O17 (∼3°), and C14−O17−H18 (∼3°). The PBE0 harmonic and anharmonic frequencies of the 48 vibrational modes of the lactim+ in its electronic ground state are listed in Table 2. These frequencies are arranged in a descending order under a′ symmetry (in-plane vibrations) and a″ symmetry (out-of-plane vibrations). Figure 2 presents the evolution on the neutral (singlet) and the cationic (doublet) 3-hydroxyisoquinoline lowest potential energy surfaces along the lactim−lactam H transfer coordinate. These calculations are done at the PBE0/6-311G+(d,p) and (R)CCSD(T)-F12/cc-pVTZ-F12 including the CV and SR corrections using the equilibrium structures optimized at PBE0 level. Our large explicitly correlated calculations confirm the neutral lactim (enol) form is more stable than the neutral lactam (keto) form by about 0.30 eV, which is in good agreement with previous experimental and theoretical studies.6−8 In contrast, the lactam+ (keto) form is found to be more stable than lactim+ (enol) by ∼0.27 eV. The prominent stability of the lactam forms after ionization was also noticed and observed for 2-pyridone+ cation and its derivatives.9,42 The neutral lactim may isomerize into lactam by proton transfer from O17 to N16. At the (R)CCSD(T)-F12/cc-pVTZ-

neutral lactim (X̃ 1A′)

lactim+ (X̃ 2A″)

1.345 1.346 1.376 1.407 1.418 1.370 1.415 1.369 1.415 1.424 1.411 1.312 0.964

1.339 1.303 1.423 1.389 1.422 1.383 1.397 1.401 1.390 1.429 1.428 1.309 0.972

1.084 1.086 1.085 1.085 1.086 1.089 115.9 119.9 106.3 124.3 118.3 118.1 117.3 124.0 117.9 118.4 120.3 121.1 119.9 120.2 120.0

1.085 1.085 1.084 1.085 1.086 1.089 118.1 117.2 109.4 124.7 118.0 117.9 118.0 124.2 117.2 119.2 120.0 120.3 120.8 120.0 119.8

lactam+ (X̃ 2A″) 1.432 1.210 1.437 1.395 1.414 1.381 1.403 1.386 1.400 1.435 1.408 1.313 1.017 1.084 1.085 1.085 1.084 1.085 1.086 118.6 126.7 114.7 121.9 119.2 118.5 121.2 124.6 118.4 120.1 121.0 120.4 119.7 120.4

F12+CV+SR level, the barrier of proton transfer is ∼1.7 eV (relative to the lactim; see Figure 2). Similarly, the lactim+ may also isomerize into lactam+ with a barrier height of ∼1.4 eV (relative to the lactim+). The (R)CCSD(T)-F12 ionization energy (IE) value (8.080 eV) of the lactim is found to be higher than that (7.509 eV) of lactam by ∼0.57 eV. On the basis of the CASSCF predictions, the dominant electronic configuration of 3-hydroxyisoquinoline lactim is (30a′) 2 (31a′) 2 (32a′) 2 (4a″) 2 (5a″) 2 (6a″) 2 . The electronic ground state X̃ 2A″ of the lactim+ is obtained by removal of one electron from the outermost (6a″) MO of the lactim. The first electronic excited state 22A″ of the lactim+ is obtained by excitation of an electron from (5a″) to (6a″) MO of the X̃ 2A″ state. The dominant electron configurations and vertical excitation energies of the lactim, lactim+, and lactam+ are listed in Tables 3, 4, and 5, respectively. At the MRCI+Q/CASSCF/ aug-cc-pVDZ level, the 22A″ state of lactim+ is computed to be 9.30 eV above the X̃ 1A′ ground state of neutral lactim. On the contrary, the first electronic excited state 12A′ of the lactam+ is as high as 9.78 eV above the neutral lactim. b. 2D Photoelectron and Slow Photoelectron Spectra. The upper trace of Figure 3 presents the full 2D spectrum of 38097



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Table 2. Harmonic and Anharmonic Frequencies (cm−1) of Doublet Lactim+ (Enol) of 3-Hydroxyisoquinoline, Obtained at the PBE0/6-311+G(d,p) Level of Theorya lactim+ (X̃ 2A″)

lactim+ (X̃ 2A″)

no.

symm

harm

anharm

assignment

no.

symm

harm

anharm

assignment

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′

3739 3233 3226 3223 3213 3208 3182 1653 1614 1596 1556 1530 1495 1443 1433 1412 1385 1300 1266 1210 1196 1177 1140 1055

3551 3116 3098 3111 3092 3080 3059 1608 1569 1559 1518 1502 1465 1414 1404 1386 1353 1280 1241 1187 1179 1142 1125 1039

ν OH ν CH ν CH ν CH ν CH ν CH ν CH δ ring, δ(COH) δ ring, ν(CO) δ ring, β CH δ ring, δ(COH) ν(CO), δ ring ν(CO), δ(COH) δ ring, δ(COH) δ ring, δ(COH) δ ring, β CH β CH, δ(COH) δ ring, β CH δ ring, β CH δ(COH), β CH δ(COH), β CH δ(COH), β CH δ ring, β CH δ ring, ν(C1C6)

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

a′ a′ a′ a′ a′ a′ a′ a′ a′ a″ a″ a″ a″ a″ a″ a″ a″ a″ a″ a″ a″ a″ a″ a″

971 927 792 759 622 532 477 423 299 1023 1002 968 910 865 776 749 699 632 497 427 401 252 175 101

955 911 784 754 616 524 471 418 296 1028 995 970 921 864 770 765 735 642 494 428 401 252 174 103

δ ring δ ring δ ring, ν(C3C4) δ ring, δ(NCO) δ ring δ ring, δ(NCO) δ ring, δ(NCO) δ(CCO), δ(NCO) δ ring, δ(NCO) γ CH γ CH γ CH γ CH γ CH γ CH γ CH, γ OH γ OH, τ ring γ OH, γ CH τ ring τ ring τ ring τ ring τ ring τ ring

Anharmonic frequencies are arranged according to the harmonic frequencies. (In plane vibration: ν stretching, β bending, δ deformation. Out of plane vibration: γ wagging, τ torsion.)

a

lactim to lactam form (via proton transfer) and followed by photoionization to lactam+ (Figure 2) can also be ruled out because only one-photon ionization is allowed by the SR photon density. This suggests that, in the present experimental conditions, the lactim to lactam intramolecular isomerization does not occur in the molecular jet and only the lactim form of 3-hydroxyisoquinoline neutral molecule is present in molecular beam. The molecular beam cooling means that the first intense transition at hυ = 8.028 eV is unlikely caused by the hot band effect and is attributed to the 3-hydroxyisoquinoline X̃ 1A′ + hυ → [3-hydroxyisoquinoline]+X̃ 2A″ + e− photoionization transition (see section IVc). This transition defines the ionization energy (IE) of 3-hydroxyisoquinoline (see below). For upper energies, several bright lines can be seen. They correspond to the vibrational and electronic cationic states, which are mainly populated by a direct photoionization process. The full SPES spectrum of 3-hydroxyisoquinoline in the photon energy range 7.4−9.8 eV is shown in the lower trace of Figure 3. It is deduced from our 2D spectrum after only considering the photoelectrons possessing kinetic energies between 0 and 100 meV. In the SPES spectrum, the signal onset is around 7.95 eV and exhibits several distinct peaks at 8.028, 8.081, 8.134, and 8.277 eV onward. The peak with the strongest intensity is at 8.081 eV while the peaks at 8.028, 8.134, and 8.277 eV are relatively weaker, especially the one at 8.028 eV. This spectrum is dominated by transitions to form the lactim+ in its electronic ground and first excited state. In addition, very intense vibrational transitions and progressions appear as well.

Figure 2. Potential energy surfaces (PESs) of neutral and cationic 3hydroxyisoquinoline at CCSD(T)-F12 level of theory with CV and SR corrections using the equilibrium structure optimized at PBE0/6311+G(d,p) level. The relative energies obtained at PBE0 level are shown in the parentheses.

hydroxyisoquinoline in the 7.4−9.8 eV photon-energy range and for photoelectrons having kinetic energies from 0 to 190 meV. In the 7.4−8 eV energy range, there is no electron signal around 7.5 eV where one may expect the formation of the lactam cationic form according to our theoretical value of 7.509 eV, predicted at the (R)CCSD(T)-F12/cc-pVTZ-F12+CV+SR level. The lack of the neutral lactam form is consistent with the energy diagram depicted in Figure 2, and the fact that the neutral species are produced in an adiabatically cooled expansion. The possibility of the tautomerization from neutral 8098



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Table 3. Dominant Electronic Configuration and Vertical Excitation Energies (eV) of the Ground and Excited States of 3Hydroxyisoquinoline (Singlet and Triplet)a state X̃ 1A′ 13A′ 21A′ 23A′ 33A′ 11A″ 13A″ 21A″ 31A″ 23A″ 33A″

electron configuration 0.97 0.96 0.92 0.66 0.70 0.66 0.80 0.61 0.74 0.81 0.91

× × × × × × × × × × ×

CASSCF

MRCI

MRCI+Q

0.00b 2.87 5.11 4.91 5.72 4.92 4.98 5.30 5.37 5.36 5.48

0.00c 3.00 5.12 5.07 5.59 5.59 5.81 5.99 6.07 6.21 6.32

0.00d 3.08 4.70 5.06 5.51 5.76 6.02 6.17 6.22 6.44 6.52

{(30a′)2(31a′)2(32a′)2(4a″)2(5a″)2(6a″)2} {(30a′)2(31a′)2(32a′)2(4a″)2(5a″)2(6a″)1(7a″)1} {(30a′)2(31a′)2(32a′)2(4a″)2(5a″)2(6a″)1(7a″)1} {(30a′)2(31a′)2(32a′)2(4a″)2(5a″)2(6a″)1(8a″)1} +0.59 × {(30a′)2(31a′)2(32a′)2(4a″)1(5a″)2(6a″)2(7a″)1} {(30a′)2(31a′)2(32a′)2(4a″)1(5a″)2(6a″)2(8a″)1} +0.58 × {(30a′)2(31a′)2(32a′)2(4a″)2(5a″)1(6a″)2(8a″)1} {(30a′)2(31a′)2(32a′)2(33a′)1(4a″)2(5a″)2(6a″)1} +0.56 × {(30a′)2(31a′)2(32a′)2(34a′)1(4a″)2(5a″)2(6a″)1} {(30a′)2(31a′)2(32a′)2(33a′)1(4a″)2(5a″)2(6a″)1} +0.50 × {(30a′)2(31a′)2(32a′)2(34a′)1(4a″)2(5a″)2(6a″)1} {(30a′)2(31a′)2(32a′)2(33a′)1(4a″)2(5a″)2(6a″)1} {(30a′)2(31a′)2(32a′)2(35a′)1(4a″)2(5a″)2(6a″)1} +0.57 × {(30a′)2(31a′)2(32a′)2(34a′)1(4a″)2(5a″)2(6a″)1} {(30a′)2(31a′)2(32a′)2(34a′)1(4a″)2(5a″)2(6a″)1} +0.50 × {(30a′)2(31a′)2(32a′)2(33a′)1(4a″)2(5a″)2(6a″)1} {(30a′)2(31a′)2(32a′)2(35a′)1(4a″)2(5a″)2(6a″)1}

The electronic states are arranged according to the MRCI+Q energies. bTotal CASSCF/aug-cc-pVDZ energy = −474.300545 Eh. cTotal MRCI/ CASSCF/aug-cc-pVDZ energy = −475.121185 Eh. dTotal MRCI+Q/CASSCF/aug-cc-pVDZ energy = −475.331664 Eh. a

Table 4. Dominant Electron Configurations Vertical Excitation Energies (eV) of the Ground and Excited States of Doublet Lactim+ (Enol) of 3-Hydroxyisoquinoline state X̃ 2A″ 22A″ 12A′ 32A″ 42A″ 22A′ 52A″ 32A′ 42A′ 52A′

electron configuration 0.92 × {(30a′)2(31a′)2(32a′)2(4a″)2(5a″)2(6a″)1} 0.88 × {(30a′)2(31a′)2(32a′)2(4a″)2(5a″)1(6a″)2} 0.90 × {(30a′)2(31a′)2(32a′)1(4a″)2(5a″)2(6a″)2} 0.80 × {(30a′)2(31a′)2(32a′)2(4a″)1(5a″)2(6a″)2} 0.85 × {(30a′)2(31a′)2(32a′)2(4a″)2(5a″)2(7a″)1} 0.74 × {(30a′)2(31a′)2(32a′)1(4a″)2(5a″)2(6a″)1(7a″)1} 0.59 × {(30a′)2(31a′)2(32a′)2(4a″)2(5a″)2(8a″)1} +0.57 × {(30a′)2(31a′)2(32a′)2(4a″)2(5a″)1(6a″)1(7a″)1} 0.90 × {(30a′)2(31a′)2(32a′)2(33a′)1(4a″)2(5a″)2} 0.90 × {(30a′)2(31a′)1(32a′)2(4a″)2(5a″)2(6a″)2} 0.91 × {(30a′)1(31a′)2(32a′)2(4a″)1(5a″)2(6a″)2}

CASSCF

CASSCFa

MRCI

MRCIa

MRCI +Q

MRCI +Qa

0.00b 1.85 3.62 3.51 4.63 6.57 6.64

5.19 7.04 8.81 8.70 9.83 11.77 11.83

0.00c 1.90 3.64 3.44 4.55 6.71 6.64

7.40 9.30 11.04 10.84 11.95 14.11 14.04

0.00d 1.75 2.71 3.29 4.14 5.75 5.88

7.55 9.30 10.26 10.84 11.69 13.30 13.43

7.03 7.07 7.59

12.22 12.27 12.78

6.95 7.46 7.56

14.35 14.86 14.96

6.19 6.78 7.46

13.74 14.33 15.01

These energies are given with respect to the X̃ 1A′ minimum of neutral 3-hydroxyisoquinoline lactim. bTotal CASSCF/aug-cc-pVDZ energy = −474.109642 Eh. cTotal MRCI/CASSCF/aug-cc-pVDZ energy = −474.849251 Eh. dTotal MRCI+Q/CASSCF/aug-cc-pVDZ energy = −475.054163 Eh. a

Table 5. Dominant Electron Configurations and Vertical Excitation Energies (eV) of the Ground and Excited States of Doublet Lactam+ (Keto) of 3-Hydroxyisoquinoline state X̃ 2A″ 2

1 A′ 22A″ 32A″ 42A″ 52A″ 22A′ 42A′ 32A′ 52A′

electron configuration 0.90 × {(31a′)2(32a′)2(4a″)2(5a″)2(6a″)1} 0.93 × {(31a′)2(32a′)1(4a″)2(5a″)2(6a″)2} 0.73 × {(31a′)2(32a′)2(4a″)2(5a″)2(7a″)1} 0.63 × {(31a′)2(32a′)2(4a″)1(5a″)2(6a″)2} 0.72 × {(31a′)2(32a′)2(4a″)2(5a″)1(6a″)2} 0.54 × {(31a′)2(32a′)2(4a″)2(5a″)1(6a″)1(7a″)1} +0.50 × {(31a’)2(32a′)2(4a″)2(5a″)2(8a″)1} 0.70 × {(31a′)2(32a′)1(4a″)2(5a″)2(6a″)1(7a″)1} 0.65 × {(31a′)2(32a′)1(4a″)2(5a″)1(6a″)2(7a″)1} 0.92 × {(31a′)2(32a′)2(33a′)1(4a″)2(5a″)2} 0.91 × {(31a′)2(32a′)2(34a′)1(4a″)2(5a″)2}

CASSCF

CASSCFa

MRCI

MRCIa

MRCI +Q

MRCI +Qa

0.00b 1.93 3.18 3.76 4.25 5.89

5.32 7.25 8.50 9.08 9.57 11.21

0.00c 2.25 3.15 3.67 4.43 6.20

7.57 9.82 10.72 11.24 12.00 13.77

0.00d 2.21 2.90 3.23 3.66 5.37

7.57 9.78 10.48 10.80 11.23 12.94

5.28 6.89 6.23 6.95

10.60 12.21 11.55 12.27

6.45 7.67 6.96 7.69

14.02 15.24 14.53 15.26

5.86 6.83 6.86 7.53

13.43 14.40 14.43 15.10

a These energies are given with respect to the X̃ 1A′ minimum of neutral 3-hydroxyisoquinoline lactim. bTotal CASSCF/aug-cc-pVDZ energy = −474.105087 Eh. cTotal MRCI/CASSCF/aug-cc-pVDZ energy = −474.842965 Eh. dTotal MRCI+Q/CASSCF/aug-cc-pVDZ energy = −475.053412 Eh.

c. Tentative Assignment of the SPES Spectrum of 3Hydroxyisoquinoline. As the molecular structures of the neutral 3-hydroxyisoquinoline lactim and its cation are significantly different; based on the Franck−Condon principle, the origin band for the ionization transition of 3-hydroxyisoquinoline is expected to be weak. Guided by the explicitly correlated calculations at the (R)CCSD(T)-F12/cc-pVTZ-

F12+CV+SR level, the peak at hυ = 8.028 eV is tentatively assigned to be the origin band for the ionization transition of lactim. The theoretical IE of 8.080 eV comes very close to the experimental IE(lactim) = 8.028 value determined in the SPES spectrum. The tentative assignment of the photoelectron peaks above 8.0 eV is primarily based on the harmonic/anharmonic 8099



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Table 6. Tentative Assignment for Observed Frequencies of Photoionization Bands from the SPES Spectrum tentative assignment

observed energy (eV)

observed frequenciesa (cm−1)

predicted frequencies

X̃ 000 (lactam+) X̃ 000 (lactim+) X̃ 3210 X̃ 3220 X̃ 3230 X̃ 32102610 X̃ 32203320 X̃ 3240 X̃ 32203330 or X̃ 32202610 X̃ 32303320 X̃ 32101310 X̃ 3210810 X̃ 3250 X̃ 3220810 X̃ 3260 or X̃ 32403330 ̃ X3270 or X̃ 3230810

7.509b 8.028 8.081 8.134 8.182 8.195 8.210 8.227 8.244

0 425 850 1240 1346 1465 1606 1736

418 836 1254 1329 1428 1672 1724, 1763

8.252 8.263 8.277 8.289 8.324 8.346

1807 1890 2008 2102 2386 2563

1846 1883 2026 2090 2444 2508, 2560

8.391

2929

2907, 2926

a

Relative to origin band of 3-hydroxyisoquinoline lactim. bCalculation result relative to ground state neutral lactam form at CCSD(T)-F12 level.

Figure 3. (upper trace) Full-scale 2D spectrum of 3-hydroxyisoquinoline providing the photoelectron kinetic energies as a function of the photon energy after the energy-shifting treatment (see ref 9). (lower trace) Slow photoelectron spectrum (SPES) (red line) deduced from the 2D spectrum.

vibrational frequencies predictions. The photoelectron peaks in the 8.0−8.4 eV range, and their tentative assignments are shown in Table 6 and Figure 4. Our quoted energies correspond to the maximum of the bands. We are quite confident in the assignments of the isolated bands, whereas the deduction may be tentative for the combination bands (e.g., peak at 8.244 eV). Both the neutral and the cation belong to Cs point group, vibrational excitation of a′ and even overtones of a″ vibrational modes are symmetry allowed upon single photon ionization. The assignments of some vibrational modes are guided by the anharmonic frequencies and the decomposition of the corresponding normal modes on the internal coordinates. We fully assign these vibrational features to the pure vibration progressions or combination modes involving the lactim cationic vibrational modes 8, 13, 26, 32, and 33 (a′ symmetry). The most intense transition (at hυ = 8.081 eV) in the SPES spectrum is assigned as ν32+ = 425 cm−1. This vibrational mode ν32+ corresponds mainly to the angular deformation (in-plane) involving the C10−C14−O17 and N16−C14−O17 angles, which represent the major geometrical differences between 3hydroxyisoquinoline lactim and lactim+. A number of overtones for ν32+ are also observed: 2ν32+ (850 cm−1), 3ν32+ (1240 cm−1), 4ν32+ (1606 cm−1), 5ν32+ (2102 cm−1), 6ν32+ (2563 cm−1), and 7ν32+ (2929 cm−1). Other peaks can be assigned as the combination band of ν32+ mode with other modes (ν8+, ν13+, ν26+, ν32+, and ν33+). For example, the peaks at 1346 and 1465 cm−1 are assigned as ν32+ + ν26+ and 2ν32+ + 2ν33+,

Figure 4. Blowup of the SPES spectrum of Figure 3 in the 7.7−8.5 eV where the comb lines correspond to the tentative assignments.

respectively; the bands at 1890 and 2008 cm−1 can be represented by ν32+ + ν13+ and ν32+ + ν8+, respectively. Some peaks may have more than one assignment, e.g., the peak at 1736 cm−1 can be assigned to 2ν32+ + 3ν33+ or 2ν32+ + ν26+, the peak at 2563 cm−1 can be either 6ν32+ or 4ν32+ + 3ν33+, and the peak at 2929 cm−1 can be 7ν32+ or 3ν32+ + ν8+. Upon analysis of the SPES spectrum, we may determine the fundamental frequency of some vibrational modes of the lactim+: ν8+ ∼1583, ν13+ ∼1465, ν26+ ∼921, and ν33+ ∼307 cm−1. Both the vibrational modes ν8+ and ν13+ are induced by the in-plane angular deformation of the C14−O17−H18 angle, while mode ν8+ involves also the in-plane ring deformation and mode ν13+ corresponds to the C14−O17 bond stretching. For vibrational modes ν26+ and ν33+, the in-plane ring deformation is the main contribution while the mode ν33+ involves the in-plane angular deformation of the N16−C14−O17 angle. 8100



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(2) Ammon, H. L.; Wheeler, G. L. The Crystal and Molecular Structures of Three Isoquinoline Derivatives: 1-Chloro-3-hydroxyisoquinoline, 1-Phenyl-2-methyl-3-isoquinolone and 2-(2′,6′-Dichlorobenzyl)-1-isoquinolone. Acta Crystallogr. 1974, B30, 1146−1154. (3) Allan, K. M.; Hong, B. D.; Stoltz, B. M. Expedient Synthesis of 3Hydroxyisoquinolines and 2-Hydroxy-1,4-naphthoquinones via Onepot Aryne Acyl-Alkylation/Condensation. Org. Biomol. Chem. 2009, 7, 4960−4964. (4) Hendricks, R. T.; et al. 3-Hydroxyisoquinolines as Inhibitors of HCV NS5b RNA-dependent RNA Polymerase. Bioorg. Med. Chem. Lett. 2009, 19, 410−414. (5) Nimlos, M. R.; Kelley, D. F.; Bernstein, E. R. Spectroscopy, Structure, and Proton Dynamics of 2-Hydroxypyridine and Its Clusters with Water and Ammonia. J. Phys. Chem. 1989, 93, 643−651. (6) Wei, C. Y.; Yu, W. S.; Chou, P. T.; Hung, F. T.; Chang, C. P.; Lin, T. C. Conjugated Dual Hydrogen-Bond Mediating Proton-Transfer Reaction in 3-Hydroxyisoquinoline. J. Phys. Chem. B 1998, 102, 1053− 1064. (7) Ramos, A. F.; Smedarchina, Z.; Zgierski, M. Z. Direct-dynamics Approach to Catalytic Effects: The Tautomerization of 3-Hydroxyisoquinoline as a Test Case. J. Chem. Phys. 2000, 113, 2662−2670. (8) Gerega, A.; Lapinski, L.; Nowak, M. J.; Furmanchuk, A.; Leszczynski, J. Systematic Effect of Benzo-Annelation on OxoHydroxy Tautomerism of Heterocyclic Compounds. Experimental Matrix-Isolation and Theoretical Study. J. Phys. Chem. A 2007, 111, 4934−4943. (9) Poully, J. C.; et al. Photoionization of 2-Pyridone and 2Hydroxypyridine. Phys. Chem. Chem. Phys. 2010, 12, 3566−3572. (10) Mahjoub, A.; et al. Slow Photoelectron Spectroscopy of δValerolactam and Its Dimer. ChemPhysChem 2011, 12, 1822−1832. (11) Briant, M.; Poisson, L.; Hochlaf, M.; de Pujo, P.; Gaveau, M.-A.; Soep, B. Ar2 Photoelectron Spectroscopy Mediated by Autoionizing States. Phys. Rev. Lett. 2012, 109, 193401−5. (12) Adamo, C.; Barone, V. Toward Reliable Density Functional Methods Without Adjustable Parameters: The PBE0Model. J. Chem. Phys. 1999, 110, 6158−6170. (13) Frisch, M. J. et al. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford CT, 2009. (14) Adler, T. B.; Knizia, G.; Werner, H.-J. A Simple and Efficient CCSD(T)-F12 Approximation. J. Chem. Phys. 2007, 127, 221106. (15) Werner, H.-J.; Knizia, G.; Manby, F. R. Explicitly Correlated Coupled Cluster Methods with Pair-specific Geminals. Mol. Phys. 2011, 109, 407−417. (16) Knizia, G.; Adler, T. B.; Werner, H.-J. Simplified CCSD(T)-F12 Methods: Theory and Benchmarks. J. Chem. Phys. 2009, 130, 054104. (17) Brites, V.; Hochlaf, M. Titan’s Ionic Species: Theoretical Treatment of N2H+ and Related Ions. J. Phys. Chem. A 2009, 113, 11107−11111. (18) Lique, F.; Klos, J.; Hochlaf, M. Benchmarks for the Generation of Interaction Potentials for Scattering Calculations: Applications to Rotationally Inelastic Collisions of C4 (X3Σ−g) with He. Phys. Chem. Chem. Phys. 2010, 12, 15672−15680. (19) Rauhut, G.; Knizia, G.; Werner, H.-J. Accurate Calculation of Vibrational Frequencies Using Explicitly Correlated Coupled-cluster Theory. J. Chem. Phys. 2009, 130, 054105. (20) Peterson, K. A.; Adler, T. B.; Werner, H.-J. Systematically Convergent Basis Sets for Explicitly Correlated Wavefunctions: The Atoms H, He, B−Ne, and Al−Ar. J. Chem. Phys. 2008, 128, 084102. (21) Weigend, F. A Fully Direct RI-HF Algorithm: Implementation, Optimised Auxiliary Basis Sets, Demonstration of Accuracy and Efficiency. Phys. Chem. Chem. Phys. 2002, 4, 4285−4291. (22) Hättig, C. Optimization of Auxiliary Basis Sets for RI-MP2 and RI-CC2 Calculations: Core−valence and Quintuple-ζ Basis Sets for H to Ar and QZVPP Basis Sets for Li to Kr. Phys. Chem. Chem. Phys. 2005, 7, 59−66. (23) Klopper, W. Highly Accurate Coupled-Cluster Singlet and Triplet Pair Energies From Explicitly Correlated Calculations in Comparison with Extrapolation Techniques. Mol. Phys. 2001, 99, 481−507.

The SPES spectrum becomes more complex in the region 8.4 to 9.8 eV. It contains several unresolved peaks arising from groups of vibrational modes of similar frequencies and most likely their progressions. A remarkable peak, superimposed onto a broad unresolved structure, is found to lie at 9.26 eV, i.e., 1.23 eV above the X̃ 000 band of lactim+. The energy of this peak is in good accordance with the excitation energy (9.30 eV relative to lactim; see Table 4) of the electronically excited state (22A″) of lactim+ at the MRCI+Q level. According to the theoretical results, this region of the spectrum should correspond to the photoionization transitions populating high vibrational levels of the lactim+ X̃ 2A″ and 22A″. For hυ > 9.8 eV, the SPES spectrum (not shown here) is structure-less because of congestion of the bands resulting from the ionization toward the upper vibrational bands of the X̃ 2A″ and 22A″ states and because of the prominent contribution of predissociation processes and vibronic interaction between closely lying cationic states.

V. CONCLUSIONS We present a combined theoretical and experimental study of the 3-hydroxyisoquinoline+ cation spectroscopy and energetics. The latter were deduced using state-of-the-art theoretical methodology, which allows direct comparison to the most accurate experimental data. Experimentally, the SPES spectrum of this molecule consists of well-resolved rich and complex features. Although the lactam+ isomer is more stable, the SPES spectrum was assigned solely to the vibrational bands of the lactim+ form, because of high tautomerization barriers both in the neutral and the ionic form of the lactim tautomer, the only one produced as a neutral in the molecular beam. General good agreement is found between the present experimental and theoretical determinations.

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ASSOCIATED CONTENT

S Supporting Information *

Full lists of coauthors for refs. 4, 9, 10, 13, and 36. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +33160957319. Fax: +33160957320. Electronic mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are indebted to the general technical staff of Synchrotron Soleil for running the facility. We would like also to thank JeanFrançois Gil for his technical help on the SAPHIRS molecular beam chamber. N. Nieuwjaer, F. Lecomte, G. Grégoire, and B. Manil are thanked for their help during collecting the data at Synchrotron Soleil. M.H. would like to thank PCMI (INSU, CNRS) for financial support. K.-C.L. thanks the Université Paris-Est Marne-la-Vallée for a visiting fellowship during the preparation of this work. The theoretical work was supported by a Strategic Research Grant from City University of Hong Kong (K.-C.L., Project No. 7002676).

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REFERENCES

(1) Evans, D. A.; Smith, G. F.; Wahid, M. A. The Tautomerism of 3Hydroxyisoquinolines. J. Chem. Soc. B 1967, 590−595. 8101



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