Infrared Spectroscopy of Solvation in Small Zn+(H2O)n Complexes

Article pubs.acs.org/JPCA

Infrared Spectroscopy of Solvation in Small Zn+(H2O)n Complexes Biswajit Bandyopadhyay, Kimberly N. Reishus, and Michael A. Duncan* Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States S Supporting Information *

ABSTRACT: Singly charged zinc-water cations are produced in a pulsed supersonic expansion source using laser vaporization. Zn+(H2O)n (n = 1−4) complexes are mass selected and studied with infrared laser photodissociation spectroscopy, employing the method of argon tagging. Density functional theory (DFT) computations are used to obtain the structures and vibrational frequencies of these complexes and their isomers. Spectra in the O−H stretching region show sharp bands corresponding to the symmetric and asymmetric stretches, whose frequencies are lower than those in the isolated water molecule. Zn+(H2O)nAr complexes with n = 1−3 have O−H stretches only in the higher frequency region, indicating direct coordination to the metal. The Zn+(H2O)2−4Ar complexes have multiple bands here, indicating the presence of multiple low energy isomers differing in the attachment position of argon. The Zn+(H2O)4Ar cluster uniquely exhibits a broad band in the hydrogen bonded stretch region, indicating the presence of a second sphere water molecule. The coordination of the Zn+(H2O)n complexes is therefore completed with three water molecules.

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INTRODUCTION Solvated metal ions govern many significant aspects of chemical and biological processes in aqueous solutions.1−3 Gas phase metal ion−water complexes provide model systems for these solvation processes at the molecular level.4−10 These systems have been studied with mass spectrometry to determine binding energies, coordination trends, and reactions.4−24 Computational chemistry has investigated hydration structures and energetics, coordination numbers, and the development of solvation spheres. 25−39 Spectroscopic studies of cation hydration first explored electronically excited states,40−49 but more recent measurements of infrared spectroscopy have investigated solvation in the ground state.50−67 Transition metal and main group ions have been studied as well as multiply charged systems. In the present work, we extend these infrared studies to the Zn+(H2O)n system. Metal cation−water complexes have been produced and studied in the gas phase using mass spectrometry for over 30 years. Collision induced dissociation and equilibrium measurements have provided ion−solvent bonding energies.4−24 Experiments have also used photodissociation of mass-selected clusters to determine fragmentation channels and coordination numbers.40−49 Early experiments were limited to singly charged species because they are easier to produce in the gas phase, but recent new ion sources extend these studies to multiply charged systems.49,58−61,66,67 Numerous computational studies have investigated cation-water interactions,25−39 but direct experimental determinations of solvation structures are still quite limited. Electronic spectroscopy has been applied to these systems, most often using mass-selected photodissociation methods, and studying primarily on small complexes.40−49 Alkaline earth cation-water systems, M+(H2O)n (M = Mg, Ca, © 2013 American Chemical Society

Sr, Ba), which have a single valence electron and relatively simple electronic structure, were one focus of these studies.41−45 Transition metal cation−water complexes, including Zn+(H2O), were also studied using the same methods.40,46−48 In the small M+(H2O)n complexes, vibrationally resolved electronic transitions were detected and some rotationally resolved bands were analyzed, providing detailed structural information. Unfortunately, larger M+(H2O)n clusters usually produced broad, featureless spectra due to the effects of predissociation and exited state reactions, and little or no information about structures or coordination could be obtained.5,41−49 ZEKE photoelectron spectroscopy has also been applied to study neutral metal−water complexes for metals with low ionization energies, providing some vibrational levels in the ground electronic states of the corresponding cations.68,69 In recent work, infrared photodissociation (IRPD) spectroscopy has been employed to study the structures of metal ion complexes as a function of progressive solvation.50−67 In combination with computational studies, IR spectroscopy can be used to measure the frequency shifts of solvent vibrational bands induced by cation bonding, the structures of cationsolvent complexes, and the coordination numbers of cations. Lisy and co-workers first employed this method to study alkalimetal complexes with water.50,51 Our research group and others have extended these studies to main group and transition metal−water systems.52−67 Because cation−water binding energies are substantial, the smallest hydrated complexes are Received: May 11, 2013 Revised: July 9, 2013 Published: July 22, 2013 7794

dx.doi.org/10.1021/jp4046676 | J. Phys. Chem. A 2013, 117, 7794−7803

The Journal of Physical Chemistry A

Article

Table 1. Binding (B.E.) for Water or Argon, Relative Energies (R.E.), and the DFT Computed and Experimental O−H Stretches of Various Zn+(H2O)nAr Complexes and Their Isomersa complex

a

isomer

B.E.

R. E.

theory

Zn+(H2O) Zn+(H2O)Ar Zn+(H2O)Ar Zn+(H2O)2

1a 1b 2.1

34.6 1.4 1.2 24.8

0.0 0.0 +0.2 0.0

Zn+(H2O)2

2.2

21.3

+3.5

Zn+(H2O)2Ar

2a

0.9

0.0

Zn+(H2O)2Ar

2b

0.4

+0.5

Zn+(H2O)2Ar

2c

0.8

+3.6

Zn+(H2O)3

3.1

20.4

0.0

Zn+(H2O)3

3.2

19.3

+1.1

Zn+(H2O)3

3.3

18.1

+2.3

Zn+(H2O)3Ar

3a

0.7

0.0

Zn+(H2O)3Ar

3b

0.6

0.0

Zn+(H2O)3Ar

3c

0.1

+0.6

Zn+(H2O)3Ar

3d

0.6

+1.2

Zn+(H2O)3Ar

3e

0.1

+1.7

Zn+(H2O)3Ar

3f

0.2

+2.7

Zn+(H2O)4

4.1

17.0

0.0

Zn+(H2O)4

4.2

15.9

+1.1

Zn+(H2O)4Ar

4a

0.5

0.0

Zn+(H2O)4Ar

4b

0.5

+0.1

Zn+(H2O)4Ar

4c

0.1

+0.5

Zn+(H2O)4Ar

4d

0.5

+1.2

Zn+(H2O)4Ar

4e

0.4

+1.3

Zn+(H2O)4Ar

4f

0.1

+1.6

3574 (127), 3661 (244) 3579 (117), 3666 (232) 3522 (421), 3637 (285) 3587 (104), 3591 (67), 3674 (191), 3681(193) 2815 (2156), 3636 (74), 3653 (162), 3723 (171) 3570 (254), 3585 (75), 3667 (259), 3672 (185) 3587 (94), 3596 (70), 3673 (189), 3689 (194) 2859 (2170), 3638(72), 3657 (150), 3725 (168) 3178 (1700), 3756 (65), 3806 (63), 3821 (115), 3852 (178), 3899 (153) 3596 (48), 3596 (71), 3598 (74), 3687 (157), 3687 (160), 3690 (160) 3388 (82), 3430 (960), 3611 (43), 3661 (229), 3682 (188), 3689 (159) 3389 (80), 3432 (948), 3611 (47), 3649 (427), 3682 (186), 3689 (157) 3370 (127), 3417 (1037), 3590 (99), 3670 (211), 3676 (299), 3680 (110) 3388 (95), 3431 (950), 3611 (40) 3681 (338), 3683 (73), 3689 (158) 3583 (145), 3598 (44), 3599 (81) 3677 (203), 3690 (157), 3690 (159) 3598 (14), 3598 (91), 3599 (87) 3690(151), 3690 (164), 3690 (160) 3044 (1710), 3597 (63), 3645 (63), 3658 (114), 3690 (185), 3734 (153) 3417 (73), 3453 (854), 3609 (38), 3610 (23), 3681 (253), 3683 (81), 3692 (142), 3710 (162) 3171 (1400), 3601(42), 3603 (67), 3648 (53), 3663 (99), 3695 (148), 3696 (151), 3738 (140) 3414 (74), 3451 (925), 3605 (79) 3609 (24), 3680 (245), 3682 (85), 3687 (197), 3710 (161) 3427 (68), 3462 (845), 3599 (87), 3617 (35), 3680 (251), 3683 (72), 3697 (224), 3698 (171) 3425(72), 3461 (835), 3609 (28), 3617 (35), 3680 (257), 3682 (74) 3697(145), 3709 (163) 3174 (1411), 3598 (90), 3601 (63), 3647 (53), 3663 (98), 3688 (192), 3695 (148), 3737 (140) 3153 (1497), 3601 (41), 3602 (68), 3643 (82), 3663 (98), 3695 (147), 3696 (149), 3732 (179) 3169 (1400), 3601 (38), 3603 (70), 3648 (54), 3662 (98), 3696 (146), 3696 (151), 3738 (140)

experiment 3567, 3645, 3727, 3755

isomer structure 1C 1C; Ar/Zn+ 1C; Ar/H2O 2C 1C; exH2O

3546, 3578, 3653, 3668

2C; Ar/H2O 2C; Ar/Zn+ 1C; exH2O; Ar/Zn+ 2C; AA-H2O 3C 2C; exH2O

3567, 3585, 3625, 3656, 3671

2C; AA-H2O; Ar/H2O 2C; AA-H2O; Ar/AA-H2O 2C; AA-H2O; Ar/Zn+ 3C; Ar/H2O 3C; Ar/Zn+ 2C; exH2O; Ar/Zn+ 3C; AA-H2O

3C; exH2O

3426, 3594, 3606, 3640, 3664, 3686

3C; AA-H2O; Ar/AA-H2O

3C; AA-H2O; Ar/H2O

3C; AA-H2O; Ar/Zn+

3C; exH2O; Ar/H2O

3C; exH2O; Ar/exH2O

3C; exH2O; Ar/Zn+

Calculated intensities are shown in parentheses in km/mol. Energies are in kcal/mol, and frequencies are in cm−1.

studied with the method of “tagging” using rare gas atoms such as argon.70−77 Larger Mn+(H2O)n clusters are studied by the

elimination of second sphere water molecules, providing detailed insight into hydrogen bonding networks and 7795



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coordination numbers.50−67 In one of the first studies on multiwater complexes, the IR spectra of the Ni+(H2O)n complexes indicated that Ni+ coordinates four water molecules in its first solvation sphere.55 Another study by Furukawa et al. found a coordination of three water molecules for the Co+(H2O)n system.65 In experiments using an electrospray ionization source (ESI) source, Williams and co-workers found that the doubly charged Zn2+(H2O)n species has a coordination number of five.67 In the present work, we investigate solvation in the corresponding singly charged Zn+(H2O)1−4 complexes.

The dissociation energies of Zn+(H2O)1−2 complexes were investigated computationally by Bauschlicher and co-workers.25,26 They predicted that the second water molecule has a lower binding energy (23.0 kcal/mol) than the first (34.5 kcal/ mol). We computed the dissociation energies of water molecules of Zn+(H2O)n complexes to be 19−35 kcal/mol (Table 1), consistent with the earlier work. Because these dissociation energies are greater than the photon energy in the O−H stretch region (3000−4000 cm−1), photodissociation of these pure Zn+(H2O)n complexes is not expected to occur with a single infrared photon. Multiphoton dissociation is not efficient with our OPO laser energy (5−10 mJ/pulse in the 3000−4000 cm−1 region, unfocused). Therefore, we used the method of argon tagging for efficient photodissociation.70−77 Argon is expected to bind rather weakly to Zn+, as demonstrated by the Zn+-Ar diatomic binding energy of 2706 cm −1 (7.74 kcal/mol) determined from its electronic spectrum.83 We compute the Zn+-Ar diatomic bond energy to be De = 4.8 kcal/mol (1680 cm−1) and the argon binding energies in these water complexes to be 0.1−1.4 kcal/mol (35− 490 cm−1). Therefore, although these predicted values may be underestimated, argon complexes of hydrated Zn+ cations should photodissociate in the O−H stretch region by elimination of argon, and we detect this process for all cluster sizes studied here. Figure 1 shows the IR spectra of the Zn+(H2O)1−4Ar complexes, each measured in the mass channel corresponding

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EXPERIMENTAL SECTION Singly charged Zn+(H2O)n ions and their complexes with argon are produced in a pulsed-nozzle laser vaporization source described previously.78 In the supersonic expansion, argon is used as buffer gas, and a few drops of water are added in the gas flow to produce the desired complexes. Experience indicates that this procedure produces a lower than saturated partial pressure of water, which is more effective for complex formation in many systems. A rotating and translating zinc rod is ablated by the third harmonic (355 nm) of a Nd:YAG laser (Spectra Physics INDI). The molecular beam is collimated by a skimmer and then cations are pulse-extracted into a reflectron time-of-flight mass spectrometer located in a differentially pumped chamber.79 Different configurations of this instrument allow measurement of the full mass spectrum or size-selection of a particular cluster ion. Selected ions are investigated with infrared photodissociation spectroscopy in the O−H stretching region using an infrared optical parametric oscillator/amplifier system (OPO/OPA; LaserVision, Inc.)80 pumped by a Nd:YAG laser (Continuum 8010). Laser excitation occurs in the turning region of the reflectron field, where ion optics and pulse timing are adjusted to optimize the overlap between the laser and the ion beam. When the laser excitation is on resonance with O−H stretch vibration, absorption occurs and then energy flows into other vibrations via intramolecular vibrational relaxation (IVR). A fraction of the ions then fragment by losing argon. The fragment ion intensity is recorded as a function of the infrared laser frequency using a digital oscilloscope (LeCroy) connected to a PC. IR wavelengths are calibrated using photoacoustic spectroscopy of methane in the region of its C−H stretch vibration. Density functional theory (DFT) is employed to investigate the structures, energetics, and infrared spectra of the Zn+(H2O)nAr (n = 1−4) complexes for comparison to the experiment. These computations use the B3LYP functional in the Gaussian 03W package and the 6-311+G(d,p) basis set.81 Although DFT is not expected to be highly accurate for the energetics of weak bonding, we have found in previous studies on related systems that it does locate appropriate low energy structures and describes their vibrational structure reliably.52−61 The computed vibrational frequencies are scaled by a factor of 0.9575, which is the recommended value for the B3LYP/6311+G(d,p) method.82

Figure 1. Infrared photodissociation spectra measured for the Zn+(H2O)1−4Ar complexes in the mass channel corresponding to the elimination of Ar. The red dashed lines correspond to the symmetric and asymmetric stretches of the isolated water molecule (3657 and 3756 cm−1, respectively).

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to the elimination of argon. The spectrum of Zn+(H2O)Ar has three main bands at 3567, 3645, and 3727 cm−1, and a weaker one at 3755 cm−1. The spectra of Zn+(H2O)2Ar and Zn+(H2O)3Ar each have two sets of closely spaced doublets at about the same positions (∼3550/3580, ∼3650/3670 cm−1). Zn+(H2O)3Ar also has an additional broad region of very weak signal near 3400 and a sharper weak band at 3625 cm−1. The spectrum of Zn+(H2O)4Ar has similar doublets at high

RESULTS AND DISCUSSION Photodissociation measurements require that the infrared photon energies exceed bond energies in the complexes under study. The binding energy of the Zn+(H2O) complex was measured previously to be 39.0 kcal/mol via collision induced dissociation experiments.18 To our knowledge, binding energies for larger Zn+(H2O)n clusters have not been measured. 7796



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throughout this study for comparison to the experiment. The O−H stretches of isomer 1a are predicted at 3579 and 3666 cm−1. The argon interaction with the O−H in isomer 1b shifts its stretches to lower frequencies at 3522 and 3637 cm−1. The predicted frequencies for isomer 1a are within 20 cm−1 of the observed bands, and the relative intensities match the experiment more closely. Therefore, we assign the 3567 and 3645 cm−1 bands to the O−H stretches of this isomer. There is no signal at lower frequencies corresponding to the argonbound O−H stretch; even though isomer 1b is computed to be close in energy, it is apparently not detected. The O−H stretches detected here for Zn+(H2O)Ar are 90 and 111 cm−1 lower than the corresponding vibrations of the isolated water molecule. Such red-shifted vibrations are well documented in previous studies50−67 and are caused by polarization of water induced by the metal cation. Although the HOMO of water involves mainly the nonbonding lone pairs on oxygen, this orbital has partial bonding character along the O−H bonds. Cation polarization removes electron density here, reducing the bond strength, and shifting the vibrations to lower frequencies. Table 2 compares the red shifts of these

frequency but is significantly different from those of the other complexes with an intense broad band centered at 3426 cm−1. This band is in the region of hydrogen bonded stretching vibrations. Previous experience with metal−water systems indicates that multiplets in the O−H stretching region such as those seen here for the n = 2−4 complexes may arise from the presence of more than one isomeric structure. To investigate this, we used computational studies on the structures, isomers, and spectra for these various complexes. All of the low energy structures were found to have doublet electronic states arising from the Zn+ (3d104s1) atomic configuration, which is considerably lower in energy than the excited 3d104p1 or 3d94s2 states.84 As shown in Table 1, different binding sites of water and argon produce various low energy isomers for several of these complexes, and these isomers have different spectra in the O−H stretching region. The energetics as well as predicted (scaled) and experimental vibrational frequencies corresponding to each complex are also reported in Table 1. Figure 2 shows the spectrum of Zn+(H2O)Ar measured in the mass channel corresponding to the elimination of argon,

Table 2. O−H Stretch Frequencies and Red Shifts (In Parentheses) with Respect to Isolated Water Vibrational Bands Measured for Selected Metal Cation Water Complexesa complex H2O49 Li+(H2O)Ar Mg+(H2O)Ar Al+(H2O)Ar Sc+(H2O)Ar V+(H2O)Ar Cr+(H2O)Ar Mn+(H2O)Ar Fe+(H2O)Ar Co+(H2O)Ar3 Ni+(H2O)Ar2 Cu+(H2O)Ar2 Zn+(H2O)Ar

symmetric stretch 3657 3629 3579 3464 3580 3605 3620 3584 3615 3624 3623 3623 3567

(28) (78) (193) (77) (52) (37) (73) (42) (33) (34) (34) (90)

asymmetric stretch 3756 3691 3650 3587 3656 3690 3690 3660

(65) (106) (169) (100) (66) (66) (96)

3697 3696 3696 3645

(59) (60) (60) (111)

a

Our work32 except as noted. All complexes here have the argon tag atom(s) bound on the metal cation and not on the OH of water. All units are cm−1.

Figure 2. IR spectrum of the Zn+(H2O)Ar complex, measured in the mass channel corresponding to the elimination of Ar (top trace). The bottom two traces show the predicted spectra and structures for two low energy isomers. The red dashed lines correspond to the symmetric and asymmetric stretches of the isolated water molecule (3657 and 3756 cm−1, respectively).

vibrations to those of other cation−water complexes. As shown, the magnitude of the red shift for Zn+(H2O)Ar is the greatest of the first row transition metal cation−water complexes. Singly charged Sc+(H2O)Ar and Mn+(H2O)Ar also have large red shifts (77/100 and 73/96 cm−1, respectively), but for Cu+(H2O)Ar2, Cr+(H2O)Ar, and Co+(H2O)Ar3 these shifts are much smaller (34/60, 37/66, and 33/59 cm −1 , respectively). The other general feature of the spectra of M+(H2O) complexes is that the intensity pattern of the O−H stretches is quite different from that in the isolated water molecule. The intensity ratio of these stretches changes from 1:18 in free water86 to about 1:2 in Zn+(H2O)Ar. The symmetric stretch in a metal cation−water system is a parallel-type vibration that modulates the charge more effectively along the axis of the dipole moment than the perpendicular-type asymmetric stretch. This leads to a greater change in the dynamic dipole than that of the asymmetric stretch. Therefore, although both O−H

and its comparison to the predictions of theory. The red dashed lines correspond to the symmetric and asymmetric stretches of the isolated water molecule (3657 and 3756 cm −1 , respectively).85 This complex is computed to have the Zn+water moiety in a C2v configuration, as determined previously.48 There are two low lying argon isomers, the lowest energy one with argon on Zn+ (isomer 1a) and another with argon on O− H (1b). These are predicted to be separated by only 0.2 kcal/ mol, and their predicted spectra are shown in the bottom two traces of Figure 2. The structure with argon on the C2 axis opposite water is not a minimum and relaxes to isomer 1b. We use a Lorentzian line width of 10 cm−1 (matching the experimental line width) to plot the predicted spectra 7797



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stretches have higher intensities in cation−water complexes than they do in the water molecule (see the computed intensities in the Supporting Information), the symmetric stretch gains more intensity than the asymmetric stretch. This effect has also been documented for other metal cation−water complexes.52−61 The assignment of the 3727 and 3755 cm−1 bands detected here is not obvious from the harmonic calculations. The 3727 cm−1 feature is located 61 cm−1 higher than the predicted asymmetric stretch. However, bands in this region have been seen previously for a number of cation−water−argon complexes, resulting from two general types of behavior.52−61 In most of these systems, the argon binds to the metal ion on the C2 axis opposite the water giving the structure C2v symmetry. Because all of the heavy atoms lie on the C2 axis, the moment of inertia around this axis is then only affected by the light hydrogen atoms. The A rotational constant is therefore close to its value for the free water molecule (13 to 14 cm−1), resulting in partially resolved K-type rotational structure for the asymmetric stretch band. This kind of rotational structure has been analyzed for the V+, Li+, Cr+, and Sc+ complexes, providing their structures and rotational temperatures.52,56,58,59 In other complexes when the Ar binds off the C2 axis, the A constant is much smaller, and the rotational structure is not detected at our resolution.53−55,57,60,61 As shown in the inset structure of Figure 2, Zn+(H2O)Ar falls into this second category, forming the 1a structure with argon off the C2 axis. The reason for this asymmetric structure has been discussed previously in studies of Mg+(H2O)Arn and Mn+(H2O)Arn.54,60 In each of these systems, the s1 electronic configuration in the ground state complex produces an s orbital that is highly polarizable. This orbital can be back-polarized by water, producing a lobe of negative charge on the C2 axis opposite its binding site, effectively becoming sp hybridized.54,60 The argon avoids this region and binds off the axis to the side of the metal ion. The 2S ground state of the Zn+ ion (3d104s1) is similar to those of Mg+ (2p63s1) and Mn+ (3d54s1) and apparently exhibits this same effect. This is why the argon binds off the axis and no rotational structure is detected on the asymmetric stretch vibrational band. A similar effect is found for complexes tagged with two or more argons, where heavy atoms are necessarily positioned off the C2 axis.52−61 Although no widely spaced rotational structure is detected, systems like this often have one or more additional vibrational bands at frequencies higher than the O−H stretches. These bands have been assigned to combinations between the asymmetric stretch and a torsional bending mode, which can be viewed as frustrated rotations.57 The higher frequency region here for Zn+(H2O)Ar looks very similar to patterns seen previously. Therefore, we assign the bands at 3727 and 3755 cm−1 to this kind of stretch-torsion combinations. The interval of the first such combination band is 82 cm−1 above the asymmetric stretch band, with which it is most likely to couple.57 Although this interval does not match the computed torsional mode frequencies (52 and 62 cm−1), such modes may exhibit significant (negative) anharmonicity not captured by the harmonic theory. Figure 3 shows the experimental spectrum of Zn+(H2O)2Ar along with those predicted for its two low energy isomers. The structures of these isomers, resulting from argon attachment on-metal versus on-water, are shown in the insets. The IR spectrum has two sets of doublets at 3546/3578 and 3653/ 3668 cm−1. A comparison of this spectrum with those predicted

Figure 3. IR spectrum of the Zn+(H2O)2Ar complex (top trace) with the predicted spectra and structures corresponding to the ArZn+(H2O)2 and Zn+(H2O)2Ar isomers.

suggests that both isomers may be present. Isomer 2b is not the lowest in energy, but it is the simpler structure with argon attached to the metal ion, so we discuss it first. Its two symmetric (3587/3596 cm−1) and asymmetric O−H stretches (3673/3689 cm−1) are predicted to be quite close to each other. We can therefore assign the experimental bands at 3578 and 3653/3668 cm−1 to the O−H stretches of this isomer; these have about the right symmetric/asymmetric spacing and are shifted less from the positions of free water, as expected for a structure with argon on the metal. Isomer 2a has argon bound on an O−H of water, breaking the symmetry of the complex. This perturbation on the hydrogen motion shifts the stretches to lower frequencies, with the symmetric stretches more affected than the asymmetric. The symmetric stretch at 3570 cm−1 is about 20 cm−1 red-shifted from the corresponding stretches of isomer 2b. On the other hand, the asymmetric stretch is predicted to lie only 4 cm−1 to the red of the nearest corresponding 2b band. The free O−H stretches (away from the argon) of isomer 2a are predicted at 3585 and 3671 cm−1. Therefore, it seems reasonable to assign the band at 3546 cm−1 to the most red-shifted O−H stretch of isomer 2a. The predicted free O−H stretches (3585/3669 cm−1) of this isomer are close to the experimental peaks at 3578/3653 cm−1 for isomer 2b and may overlap with these. It is also possible that the band spacings here are not reproduced reliably by theory. In any event, it is clear from the spectrum that both waters are attached to the metal ion in a two-coordinate (2C) structure. The IR spectrum of Zn+(H2O)3Ar is shown in the upper trace of Figure 4. The predicted structures and spectra of the selected low energy isomers of this complex are shown in the lower four traces. (A full list of these is provided in the Supporting Information.) Isomers 3a, 3b, and 3c (not shown here; see the Supporting Information) each have a 2C+1 structure, with a second-sphere water attached in a double acceptor (AA) configuration, whereas isomers 3d and 3e have 3C structures with all water molecules attached directly to the Zn+ cation. Isomers 3a, 3b, and 3c differ only in the attachment 7798



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Figure 4. IR spectrum of the Zn+(H2O)3Ar complex in the O−H stretch region (top). The predicted spectra and structures of four selected low-lying isomers are shown in the bottom traces.

Figure 5. IR spectrum of the Zn+(H2O)4Ar complex in the O−H stretch region (top). The predicted spectra and structures of four selected low-lying isomers are shown in the bottom traces.

position of argon (on-internal water, on-AA-water, on-metal), all of which have similar spectra. Even though these are predicted to be more stable, there is little evidence for this kind of isomer. They each have a strong hydrogen bonding band near 3410−3430 cm−1, and we see only a trace of broad signal in this region. Therefore, we focus instead on 3C isomers, which differ only in the attachment position of the argon (3d: on-water; 3e: on-metal). When the argon is on the metal, single bands are predicted for the symmetric and asymmetric stretches, with smaller red shifts from free-water. However, when argon is on water, the red shifts are greater and the O−H stretches split into doublets. The experimental bands have similar closely spaced doublets at 3567/3585 and 3656/3671 cm−1, and these can be assigned to the pair of doublets like those predicted for the 3d isomer at 3583/3599 and 3677/ 3690. The bands for isomer 3e are quite close to those for isomer 3d, and may also be present as partially overlapping features. Therefore, we cannot exclude this isomer completely, but find that isomer 3d is sufficient to account for the main bands. The weak band at 3625 cm−1 could come from a small amount of almost any of the other isomers (e.g., symmetric stretch of 3e). However, the main pair of doublets seems to indicate that we have mostly isomer 3d. The spectrum of Zn+(H2O)4Ar is shown in Figure 5 along with the structures and predicted spectra for several low-lying isomers. The strong new band at 3426 cm−1, which is significantly red-shifted from the free−OH region of the spectrum, immediately suggests some sort of hydrogen bonding configuration in the structure. Indeed, theory finds that the most stable structures all have one second-sphere water (3C+1 coordination). Those with this outer water in a double-acceptor configuration (4a−4c) are predicted to be more stable than

those with it in a single acceptor site (4d−4f). More subtle isomer variations again result from different argon attachment sites on each of these. For the AA configurations, the strongest hydrogen bond stretches are predicted at 3451−3462 cm−1, whereas those for the single-acceptor isomers are predicted at much lower frequencies (e.g., 3174 cm−1 for 4d and 3153 cm−1 for 4e). It is therefore immediately clear that the experimental band at 3426 cm−1 is in better agreement with the doubleacceptor type hydrogen bond vibrations like those for isomers 4a, 4b, or 4c. Like other hydrogen bond resonances seen before, this band has a greater width than the free-OH vibrational bands.55 The O−H stretching region in the experimental spectrum consists of doublets at 3594/3606 and 3664/3686 cm−1. Isomers 4a and 4b have multiplets here with some similarities to the experimental spectrum. However, the spectrum of isomer 4b has excellent agreement with the doublet spacings and relative intensities and is therefore likely the most prominent species present. The other weaker feature at 3640 cm−1 could conceivably come from another isomer. However, all peaks in this range for the various structures are predicted to have low-intensity, so if they are present in sufficient quantities to contribute to the 3640 cm−1 peak, then the other corresponding bands would also be observed. This peak is more likely another stretch-torsion combination band. These infrared spectra therefore provide an informative picture of the structures, isomers, and solvation behavior of these small Zn+(H2O)n complexes. In coordination with DFT computational work, the infrared patterns for different structures involving water or argon attachment sites are identified. For the n = 1 complex, on-metal versus on-water isomers for the argon binding are suggested by theory to lie 7799



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coordination of three water molecules is less than that of Zn2+(5C)67 or Ni+ (4C)55 and the same as that of Co+.65 More relevant is the similarity between the coordination of Zn+ and that of Mg+.63 Mg+ has the same ns1 valence configuration as Zn +, in which valence orbital polarization causes the asymmetric attachment of ligand/solvent molecules, as noted above. Mg+ also exhibits the same asymmetric coordination of three water molecules in its inner sphere with the fourth in its second shell.63 Lower coordination numbers have been seen previously for almost every metal ion studied that has a lower charge state, and consequently greater orbital occupancy, than that found typically in solution.52−61 Apparently, solvent− electron repulsion with occupied valence orbitals on the metal cause these lower coordination numbers. In the case of Zn+, the structures that result have all three solvent molecules on the same side of the metal ion in a pyramidal configuration. This is the same behavior see before for Mg+ and Al+ ions.54,62,63 As noted above, the red shifts of the O−H stretches for Zn+(H2O) Ar are greater than those of the other transition metal ion systems and comparable to the shifts seen for Mg+ and Al+ complexes.54,62,63 This red shift can be attributed to greater polarization of the water molecule, which can be associated with the size of the metal ion core that is interacting with the ligand after the valence orbital is polarized. The Zn2+ ionic radius (60 pm) is smaller than those of the other transition metal M2+ or M3+ species,84 explaining this greater polarization. Like other cation-water complexes, the red shifts of the vibrations here are gradually less as more solvent molecules are added around the metal and this polarization interaction is distributed.50−67 This study of Zn+ confirms again that metal ion electronic structure is the primary consideration in cation solvation. The picture of nascent solvation here is easy to extrapolate to the next phase, where the singly charged metal ion eventually would become the solvated doubly charged species with either a corresponding solvated electron or hydroxide ion. Recent IR studies of doubly charged metal ion complexes with water have detected the formation of such hydroxide species.58−61 Studies on larger zinc−water clusters in the future may be able to do the same, revealing the further evolution of solvation in this system.

extremely close in energy, but only the on-metal (1a) structure is identified in the experiment. The n = 2 complex has similar on-metal versus on-water isomers for argon positions, with both water molecules coordinated directly to the metal. The argon isomers in this case are also predicted to lie extremely close in energy, and evidence is found for both species in the experiment. The n = 3 complex is the first to have watercoordination isomers predicted by theory to lie close in energy. Structures with two waters coordinated to the metal and a third in a double-acceptor position (2C+1) are predicted to lie lower in energy, but there is only very weak signal in the region of hydrogen bonding vibrations, eliminating these structures as major components in the experiment. Instead, structures with three waters attached to the metal ion appear to be prominent. For these, on-metal and on-water argon isomers are predicted to lie close in energy and both appear to be present. All of the low-lying isomers predicted for the n = 4 complex have 3C+1 structures, with at least one second-sphere water molecule. Structures with the external water in a double-acceptor position are predicted to be slightly lower in energy than those with it in a single-acceptor position, and the experiment provides exclusive evidence for the AA structure. Like the other complexes studied here, additional isomers are predicted to lie close in energy for different argon attachment sites. However, the 4b isomer reproduces essentially all of the main vibrational structure observed. It is interesting throughout these different cluster sizes that the different isomers are predicted to lie extremely close in energy, with many within 1 to 2 kcal/mol of each other. This is understandable for the argon attachment isomers but is somewhat surprising for the water isomers. For the n = 1 and 2 complexes, direct attachment of water to the metal ion is predicted and observed. However, for the n = 3 species, a 2C+1 isomer is predicted to lie 1.2−1.7 kcal/mol lower in energy than the 3C species, but only the 3C is observed. For the n = 4 complex, 3C+1 isomers are predicted and observed, but there is no evidence for the single acceptor isomer, which is predicted to lie only about 1 kcal/mol above the AA species. Therefore, only a single water isomer is observed for each complex even though the energetics of other water-based structures lie extremely close in energy. Part of this is undoubtedly caused by the unreliable predictions of relative energies when using DFT for hydrogen bonding interactions.87 The relative energetics are probably even worse for the argon-based isomers throughout the different cluster sizes. On the basis of our DFT value for the bond energy of Zn+-Ar, which underestimates the experimental value by about 38%, our binding energies may be significantly underestimated in these complexes. However, the experimental absence of coexisting water isomers must also indicate that the barriers between different water configurations are not particularly high and that the collisional processes here in the cluster growth provide enough annealing to find what are presumably the true lowest energy structures. Actually, these clusters find the lowest free energy structures at the finite temperature of the experiment. However, examination of computed free energies versus temperatures up to 100K (Supporting Information) show that the energy ordering does not change. Although the relative energies predicted with DFT are suspect, the vibrational patterns for specific isomers appear to be reasonably well described using the harmonic theory and standard scaling factors. The solvent coordination of the zinc cation is interesting to compare to that of other related metal ions. The inner-sphere

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CONCLUSIONS Zn (H2O)1−4 complexes produced by laser vaporization are studied with mass-selected infrared photodissociation spectroscopy and the method of argon tagging. These spectra reveal the structures, isomers, and solvation behavior of these small complexes. In coordination with DFT computations, the infrared patterns for different structures involving water coordination or argon attachment sites are identified. The smallest complexes (n = 1−3) have water attached directly to the metal ion, whereas the n = 4 complex has one external water in a double-acceptor site. Argon attachment isomers (onmetal versus on-water) are identified by theory for all cluster sizes and are detected as coexisting species for some cluster sizes. The vibrational band shifts, IR intensities, and coordination numbers for the Zn+(H2O)n complexes exhibit interesting behavior compared to other cation−water systems that can be understood on the basis of the electronic structure of the singly charged zinc cation. 7800

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(17) Armentrout, P. B. Guided Ion Beam Studies of Transition Metal-Ligand Thermochemistry. Int. J. Mass Spectrom. 2003, 227, 289−302. (18) Cooper, T. E.; Carl, D. R.; Armentrout, P. B. Hydration Energies of Zinc(II): Threshold Collision-Induced Dissociation Experiments and Theoretical Studies. J. Phys. Chem. A 2009, 113, 13727−13741. (19) Cooper, T. E.; O’Brien, J. T.; Williams, E. R.; Armentrout, P. B. Zn2+ Has a Primary Hydration Sphere of Five: IR Action Spectroscopy and Theoretical Studies of Hydrated Zn2+ Complexes in the Gas Phase. J. Phys. Chem. A 2010, 114, 12646−12655. (20) Rodriquez-Cruz, S. E.; Jockusch, R. A.; Williams, E. R. Binding Energies of Hexahydrated Alkaline Earth Metal Ions, M2+(H2O)6, M = Mg, Ca, Sr, Ba: Evidence of Isomeric Structures for Magnesium. J. Am. Chem. Soc. 1999, 121, 1986−1987. (21) Beyer, M.; Williams, E. R.; Bondybey, V. E. Unimolecular Reactions of Dihydrated Alkaline Earth Metal Dications M2+(H2O)2, M = Be, Mg, Ca, Sr, and Ba: Salt-bridge Mechanism in the ProtonTransfer Reaction M2+(H2O)2 → MOH+ + H3O+. J. Am. Chem. Soc. 1999, 121, 1565−1573. (22) van der Linde, C.; Hemmann, S.; Höckendorf, R. F.; Balaj, O. P.; Beyer, M. K. Reactivity of Hydrated Monovalent First Row Transition Metal Ions M+(H2O)n, M = V, Cr, Mn, Fe, Co, Ni, Cu, Zn, toward Molecular Oxygen, Nitrous Oxide, and Carbon Dioxide. J. Phys. Chem. A 2013, 117, 1011−1020. (23) Schröder, D.; Schwarz, H. Generation, Stability, and Reactivity of Small, Multiply Charged Ions in the Gas Phase. J. Phys. Chem. A 1999, 103, 7385−7394. (24) Poisson, L.; Pradel, P.; Lepetit, F.; Reau, F.; Mestdagh, J. M.; Visticot, J. P. Binding Energies of First and Second Shell Water Molecules in the Fe(H2O)2+, Co(H2O)2+ and Au(H2O)2+ Cluster Ions. Eur. Phys. J. D 2001, 14, 89−95. (25) Rosi, M.; Bauschlicher, C. W., Jr. The Binding Energies of One and Two Water-Molecules to the 1st Transition-Row Metal PositiveIons. J. Chem. Phys. 1989, 90, 7264−7272. (26) Rosi, M.; Bauschlicher, C. W., Jr. The Binding Energies of One and Two Water-Molecules to the 1st Transition-Row Metal PositiveIons. II. J. Chem. Phys. 1990, 92, 1876−1878. (27) Bauschlicher, C. W., Jr.; Sodupe, M.; Partridge, H. A Theoretical Study of the Positive and Dipositive Ions of M(NH3)n and M(H2O)n For M=Mg, Ca, or Sr. J. Chem. Phys. 1992, 96, 4453−4463. (28) Feller, D.; Glendening, E. D.; Kendall, R. A.; Peterson, K. A. An Extended Basis-Set Ab-Initio Study of Li+ (H2O)n, n= 1−6. J. Chem. Phys. 1994, 100, 4981−4997. (29) Feller, D.; Glendening, E. D.; Woon, D. E.; Feyereisen, M. W. An Extended Basis-Set Ab-Initio Study of Alkali-Metal Cation-Water Clusters. J. Chem. Phys. 1995, 103, 3526−3542. (30) Glendening, E. D.; Feller, D. Cation Water Interactions - The M+(H2O)n Clusters for Alkali-Metals, M=Li, Na, K, Rb, and Cs. J. Phys. Chem. 1995, 99, 3060−3067. (31) Watanabe, H.; Iwata, S.; Hashimoto, K.; Misaizu, F.; Fuke, K. Molecular-Orbital Studies of the Structures and Reactions of Singly Charged Magnesium-Ion with Water Clusters, Mg+(H2O)n. J. Am. Chem. Soc. 1995, 117, 755−763. (32) Tsurusawa, T.; Iwata, S. Theoretical Studies of Structures and Ionization Threshold Energies of Water Cluster Complexes with a Group 1 Metal, M(H2O)n (M = Li and Na). J. Phys. Chem. A 1999, 103, 6134−6141. (33) Tsurusawa, T.; Iwata, S. Electron-Hydrogen Bonds and OH Harmonic Frequency Shifts in Water Cluster Complexes with a Group 1 Metal Atom, M(H2O)n (M=Li and Na). J. Chem. Phys. 2000, 112, 5705−5710. (34) Wasserman, E.; Rustad, R. R.; Xantheas, S. S. Interaction Potential of Al3+ in Water from First Principles Calculations. J. Chem. Phys. 1997, 106, 9769−9780. (35) Pandelov, S.; Werhahn, J. C.; Pilles, B. M.; Xantheas, S. S.; Iglev, H. An Empirical Correlation between the Enthalpy of Solution of Aqueous Salts and Their Ability to Form Hydrates. J. Phys. Chem. A 2010, 114, 10454−10457.

ASSOCIATED CONTENT

S Supporting Information *

The full citation for ref 81. Additional details are provided on the DFT computations, including the structures, energetics, and vibrational frequencies for each of the structures considered. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]; Fax: 706-542-1234. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge generous support for this work from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical, Geological, and Biosciences (grant no. DE-FG02-96ER14658).

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Infrared Spectroscopy of Solvation in Small Zn+(H2O)n Complexes

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