J. Phys. Chem. 1987, 91, 2526-2529
2526
Spectrum and Structure of Complexes of Water with s-Tetrazine Christopher A. Haynam,? Cheryl Morter, Linda Young,$and Donald H. Levy* The James Franck Institute and the Department of Chemistry, The University of Chicago, Chicago, Illinois 60637 (Received: June 17, 1986) The electronic spectra of complexes of water with s-tetrazine were observed in a supersonic free jet. Rotational structure was resolved and analyses of three isotopically substituted species provided estimates of the structure of the complexes. The spectrum of the complex tetrazine-H20 could be fit equally well by two structures, both having the water on the side of the tetrazine ring in a planar configuration. The two structures differed in the orientation of the hydrogen atoms of the water molecule about a line joining the oxygen of the water with the nitrogen of the tetrazine. Comparison with other complexes and with calculations favored one of the two structures. The complex between two water molecules and tetrazine had a structure where the two water molecules were bound to each other and the resulting water dimer was hydrogen bound to the tetrazine ring. The rotational structure of the complex of three water molecules bound to tetrazine confirmed the stoichiometry of the complex but provided little structural information.
Introduction The heteroaromatic molecule s-tetrazine has been a convenient chromophore for the study of weakly bound molecular clusters. Tetrazine absorbs in the visible and therefore a high-resolution ion laser pumped dye laser can be used to study the spectroscopy of complexes of tetrazine. With the high resolution provided by these lasers, rotational structure can be observed in the electronic spectra of fairly large complexes, and analysis of this rotational structure provides a method of determining their structure. In our laboratory we have studied a large number of clusters of tetrazine bound to one or more atoms or molecules and have observed several different kinds of structures. Rare gas atoms bond in out-of-plane positions and will even stack on top of each other in the out-of-plane positions rather than bond in-plane at the periphery of the tetrazine ring. Dimers of two tetrazine molecules bonded to each other were found to have two different structures. One structure was planar and appeared to be bonded by hydrogen bonds between the hydrogen atoms of one tetrazine and the lone pair on the nitrogen atom of the other tetrazine, while the other structure was T-shaped and may have involved hydrogen bonding between the hydrogen on one tetrazine and the *-cloud on the other tetrazine. In the preceding paper' we reported the structure of complexes of HCI, a strongly hydrogen-bonding species, and tetrazine. In this case the first HC1 molecule bonded to the side of the tetrazine molecule and a second HCl molecule bonded to the first HCI. In this paper we report a second study of strongly hydrogen-bonding molecules with tetrazine, in this case clusters of water and tetrazine. As in the case of HCl, the water molecule was found to bond to the side of the tetrazine molecule through a hydrogen bond between the proton on the water and the lone pair of the tetrazine nitrogen. Larger clusters of tetrazine and water appeared to have the additional waters bound by water-water hydrogen bonds. The experimental apparatus and techniques of spectral analysis were the same as those used in the tetrazine-HC1 study and are described in the preceding paper.
Results Low-Resolution Spectra of s-Tetrazine and Water. The upper trace of Figure 1 shows the free-jet spectrum of s-tetrazine in He in the region of its n* n 0-0 band. The lower two traces show the results of seeding successively higher concentrations of water into the jet, with higher concentrations being achieved by increasing the temperature of the water through which the helium flowed. The middle trace shows a single strong band shifted +86 cm-' from the s-tetrazine 0-0 band. This feature is assigned to the 0-0 band of the one-to-one complex between s-tetrazine and n transition of s-tetrazine. water (T-H,O) based on the K * As the water seed concentration is increased, new features are observed which increase in intensity relative to the s-tetrazine 0-0
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'Present address: Lawrence Livermore Laboratory, L-266, Livermore, CA 94550. 'Present address: Physics Division, Bldg. 203, Argonne National Laboratory, Argonne. IL 60439.
0022-3654/87/2091-2526%01.50/0
band more rapidly than the T-H20 feature. These features lie +75 and +62 cm-I from the 0 4 monomer origin and are assigned to T-(H20)2 and T-(H20)3, respectively, based on this concentration dependence. As in the T-(HCl), system, the band shift rule is not followed. This suggests that there are inequivalent binding sites between the first H 2 0 subunit and subsequent subunits. Both the D2T-H20 species and the H2T-D20 species are observed and have a shift relative to the H2T origin of 101 and 98 cm-I, respectively. Rotationally Resolved Tetrazine-H20 ( T-H20). The rotationally resolved spectrum of T-H20 is shown in the upper trace of Figure 2. The overall contour is again suggestive, as was the T-HCl band, of a perpendicularly polarized transition of a nearsymmetric top. Therefore, the water subunit should lie nearly in the plane of the s-tetrazine. A detailed analysis of this spectrum has been made using three isotopically substituted species, H,C2N,-H20, D2C2N4-H20,and H2C2N4-D20. In total, 180 transitions were considered.2 The rotational constants obtained from a rigid-rotor fit of these rotationally resolved spectra are shown in Table I. All features observed in the experimental spectra have counterparts in the calculated spectra (Figure 2, lower trace). The geometric parameters used to model this complex are defined in Figure 3 and are similar to those used in the analysis of T-HC1. The ground-state separation between the oxygen and nitrogen atoms is Ro, and the change in this parameter upon electronic excitation is A& (R,,' - Rd'). The angle formed by t h e y axis and a line passing through the N and 0 is defined as Bo in the ground state of the complex. The change in this parameter upon excitation is AO,. The parameter 4 specified the deviation of the N-..H-O bond from linearity, with the change in this parameter being A4. These six parameters are sufficient to specify the relative geometry of the T-H20 complex if it is assumed to be planar with undistorted subunits. Two structures with nearly identical rotational constants are presented in Table 11. Also shown in Table I1 is the parameter R,, which specifies the movement of each atom of the H 2 0 subunit an equal amount out-of-plane and is 0.0 f 0.2 A. One other parameter, p , which specifies the out-of-plane movement of a single hydrogen, is obtained by rotating the H 2 0 about its hydrogen bonded 0-H bond. This parameter is also zero within experimental error (0 f 24'). The parameter R,...H shown in Table I1 is the separation between the N and the nearest hydrogen of the H 2 0 subunit, while the parameter ON.. .H is the angle between the y axis and the line connecting the nitrogen and nearest hydrogen atom of the H 2 0 subunit. These last two parameters are constants derived from the first six parameters of Table I1 and can be used for comparison with the T-HCI data. (1) Haynam, C. A,; Morter, C.; Young, L.; Levy, D. H. J . Phys. Chem., preceding paper in this issue. (2) Frequencies of these transitions are available as supplementary material. See paragraph at end of text regarding supplementary material.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. IO, 1987
H20-s-Tetrazine Complexes
2521
TABLE I: Rotational Constants (cm-I) and Geometry of the One-to-one Complex between s-Tetrazine and Water
H$,N,-H20' direct fit A" B"
0.2148 (1) 0.05194 (5) 0.04187 (5) 0.22000 (8) 0.05171 (5) 0.04194 (6)
C" A'
B' C'
H2C2N4-D2O0 direct fit geometry
geometry 0.2133 0.05212 0.04189 0.2188 0.05 190 0.04195
0.2135 (2) 0.4850 (6) 0.3957 (6) 0.2183 (1) 0.04836 (6) 0.03951 (6)
D2C2N4-H2O0 direct fit geometry
0.2122 0.04863 0.03957 0.2173 0.04842 0.03959
0.2047 (1) 0.05073 (4) 0.04071 (5) 0.21018 (8) 0.05049 (4) 0.04068 (5)
0.2050 0.05073 0.04067 0.21039 0.05045 0.04070
'Structure I of Table 2 assumed. Tetrozine
+
TABLE II: Geometric Parameters Derived from the Rotational Transitions of the One-to-one ComDlex between H,O and s -Tetrazine structure I structure I1
no H20
x20
2.97 (1) 0.006 (6) 71.3 (5)' 0.0 (4) 22 (6) 2 (2) 0.0 (1) 0 f 24 2.12 61.4
H 2 0 a t 40°C
x5-.=,: I
I
I
I
I
113200
(not calculated) (not calculated) 2.4 85.2
while varying first six simultaneously. bCalculated from previous parameters.
I
113150
FREQUENCY(CM-'
2.93 0.016 (5) 67.9 (5) 0.7 (4) 208 (6) 1 (2)
)
Figure 1. Low-resolution supersonic free-jet fluorescence excitation spectrum of s-tetrazine and water, in the region of the 0-0 band of the ?r* n singlet transition of s-tetrazine. The lower two traces show new spectral features and their dependence on successively higher H 2 0seed
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EXPERIMENTAL
concentrations.
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Figure 4. The experimental observed rotatioally resolved spectrum (top) of the T-(H20)2 0-0 band based on the out-of-planepolarized T* n transition of s-tetrazine. The features marked with a * are due to the helium complex T-(H20),-He. The computer-generatedcontour (bottom) was obtained by using the constants of Table 111 and specifying a
I
1.0
-
Figure 2. The experimentally observed rotationally resolved spectrum (top) of the T-H20 0-0 band based on the out-of-planepolarized ?r* n transition of s-tetrazine. The computer-generated contour (bottom) was obtained by using the rotational constants of Table I, and specifying a C-type transition and jet temperature of 0.8 K.
I H
Y Figure 3. Geometric parameters used to fit s-tetrazine-H20 structure.
Rotationally Resolved s - T e t r a ~ i n e - ( H ~ OThe ) ~ . rotationally resolved spectrum of ~ - t e t r a z i n e - ( H ~ Ois) ~shown in the upper trace of Figure 4. The perpendicularly polarized transition, which is indicated by the overall band contour, again implies a nearly planar complex.
C-type transition. The smaller mass of the H 2 0 molecule as opposed to the HCI molecule eliminates some of the severe congestion observed in the T-(HC1)2 complex, allowing many well or partially resolved transitions to be observed. The rotational constants derived after assigning 24 rotational transitions from the H2C2N4-(H20), spectrum, 22 from the H2C2N4-(D20)2spectrum, and 15 from The the D2C2N4-(H20)2spectrum are shown in Table experimental spectrum also contained the origin of the T-(H2O)2-He complex (the two central features of which are marked by asterisks) red-shifted 1.2 cm-' from T-(H20)2. This is similar to the 1.37-cm-' red shift of T-He from T. There is some overlap between the high-frequency part of the tetra~ine-(H~O)~-He spectrum and the low-frequency part of the tetra~ine-(H,O)~spectrum. The lower trace of Figure 4 is a calculated spectrum of t e t r a ~ i n e - ( H ~ Ousing ) ~ the rotational constants of Table 111, and it should be noted that there are no missing features of significant intensity in the clear region to the blue of the origin. Consistent with the observed polarization of the transition (pure C-type), we have explored various geometries for the T-(H20)2 complex, all of which keep the oxygens coplanar with the tetrazine. All geometries considered leave the out-of-plane bonding site for He unobstructed. Two general types of structures were considered: those in which the two water molecules are bonded directly to the
2528 The Journal of Physical Chemistry, Vol. 91, No. 10, 1987
Haynam et al.
TABLE 111: Rotational Constants (cm-I) and Geometric Parameters for the van der Waals Complex between s-Tetrazine and Two Water Molecules"
A"
B" C" A'
B' C'
0.1 106 (2) 0.0336 (2) 0.0258 (2) 0.1 124 (2) 0.0333 (2) 0.0260 (2)
0.1 105 0.0337 0.0259 0.1123 0.0335 0.0259
0.1040 0.0315 0.0243 0.1056 0.03 13 0.0243
0.1033 (3) 0.0325 (2) 0.0246 (2) 0.1052 (2) 0.0320 (2) 0.0248 (4)
R, = 3.0 f 0.1 A ARo = 0.04 f 0.07 A 8, = 37 i 2O AB, = -9 i i o (I
0.1079 0.0332 0.0258 0.1095 0.0324 0.0260
(6) (8) (10) (5) (6) (20)
0.1078 0.0330 0.0254 0.1096 0.0328 0.0253
80-H,..o = 37 i l o M G H , ..o = -2 i 1 0 LNOH = 8 . I o b RN...H = 2.1 Ab
Error limits shown are 1 u. bCalculated from previous parameters.
tetrazine ring and those in which the two water molecules were first bonded to each other and the resulting water dimer bonded to the ring. Of the first type of structure, two geometries can be eliminated easily, those in which the water molecules are symmetrically bonded to nitrogens in the ring which are located either para or meta to each other. In the case of the para-bonded water molecules, it is impossible to reproduce the observed spectrum. For the meta-bonded structure, each oxygen is required to be 2.4 8, from a nitrogen in order to roughly reproduce the observed spectrum. This translates to a RN...Hof 1.4 A for a linear H bond, much shorter than the typically observed 2.0-2.1 8, for (H20)2, T-H2Q, or T-HCl. For the geometry in which the two water molecules are symmetrically bonded to nitrogens located ortho to one another, it was possible to generate structures that both reproduced the spectra of all three isotopic species and maintained a chemically sensible distance between the tetrazine nitrogen and water hydrogen. The ortho-bonded structure with the best least-squares fit was one in which the oxygens are located f56' from they axis, and RN..,H is the typical 2.1 8,. Arguing against this structure is the violation of the band shift rule and the fact that this structure gave a slightly poorer fit than the structure described below in which the second water molecule is bonded to the first and not to the tetrazine ring. (The sum of the squares of the residuals is three times greater than the structure described below.) Of the second type of structure, two possibilities were considered: (1) an undistorted water dimer subunit as described by Odutola and Dyke,3 and (2) a water dimer in which the internal hydrogen bond was allowed to distort. The water dimer subunit exists in a trans configuration with a linear H bond connecting the two monomer subunits. Relative to the tetrazine molecule, the rotations and translations of the water dimer used to fit the observed spectra keep four atoms (two H's and two 0 ' s ) of the six-atom dimer in plane (Figure 5). Note that an in-plane H is available for hydrogen bonding to a nitrogen lone pair. In the first case, an undistorted water dimer subunit in which both oxygens are required to remain in the tetrazine plane, as previously mentioned, was allowed to translate and rotate freely. As in the case of T-H20, six parameters are then required to describe the relative orientation of the tetrazine and water dimer. The parameters used are analogous to those described for the T-H20 complex and are shown in Figure 5 . The best fit structure found within the aforementioned constraints is one in which the oxygen of the in-plane H 2 0 monomer is located 2.63 A from a nitrogen, and R N ..H . is 1.7 A. The sum of the squares of the residuals from this structure was approximately two times larger than for the best fit structure to be described next. In the second case, the water dimer was allowed to have a nonlinear hydrogen bond. Thus, while the internal hydrogen bond length, RO...Hin Figure 5 , was kept constant, the angle OGH...O was allowed to vary from the Oo value of the undistorted dimer. In order to keep the number of variable parameters minimized, the rotation of the entire distorted water dimer subunit about the (3) Odutola, J. A,; Dyke, T. R. J . Chem. Phys. 1980, 72, 5062.
L
I
H Figure 5. Geometric parameters used to fit T-(H20),. The parameter Ro.. +, is fixed at the value determined previ~usly.~
TABLE I V Rotational Constants (cm-') for the Complex between s-Tetrazine and Three Water Molecules (T-(H,O),)" A B C 0.02179 (4) 0.01697 (4) ground state 0.07160 (6) 0.02183 (4) 0.01709 (4) excited state 0.07144 (5) (I
Error limits quoted are 1 u.
x axis (Le. the axis perpendicular to the tetrazine plane) was
specified by requiring the free in-plane hydrogen to point to the region of highest electron density in a nitrogen lone pair. This region was placed 1.5 8, away from the nitrogen in a radial direction. The parameters used to fit this constrained structure were R,, eo, OGH...o, and their changes upon electronic excitation. The resulting best fit values of these parameters are shown in Table 111. It should be noted that only 2 out of 61 residuals were greater than 0.007 cm-I, the homogeneously broadened line width. Rotationally Resolved s-Tetrazine-(H,O),. The rotationally resolved spectrum of T-(H,O)3 is shown in the bottom trace of Figure 6. Many distinct features are evident in this spectrum. However, they are not the result of single rotational transitions. As with the T-(HC1)2 species, a number of overlapping transitions underlie each spectral feature, but unlike T-(HC1)2, the rotational constants remain large enough that the features remain sharp and intense due to the reduced mass of three water molecules and their more favorable placement relative to the rotational axes. These features are very sensitive to small changes in the rotational constants. Table IV shows the rotational constants which result from fitting this rotational contour. A large number of parameters are required in order to define the geometry of the T-(H20)3 complex. While it is impossible to uniquely define the structure of the T-(H20), complex, some geometries can be eliminated. As with the T-(H20)2 spectrum, a band red-shifted 1.25 cm-' due to a helium complex, T-(H2O),-He, is visible in the high-resolution spectrum. Along with the fact that the experimental spectrum is reproduced quite
The Journal of Physical Chemistry, Vol. 91, No. 10, 1987 2529
H20-s-Tetrazine Complexes
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EXPERIMENTAL Y
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faithfully with a pure C-type transition (Figure 6), this suggests that the third H 2 0 is not binding to the top of the ring. Again, by analogy with the T-(H20)2 analysis, there exists a structure consistent with the observed rotational constants in which the water molecules bind symmetrically to the tetrazine ring at chemically reasonable distances. The only real argument against this structure is the violation of the band shift rule. Finally, structures exist in which the third H 2 0 interacts primarily with the other two such that neither violate the band shift rule nor interfere with the binding site for He. An example of such a structure is shown in Figure 7.
Discussion s-Tetrazine-H20. The proposed structure of s-tetrazine-H20 is given in Table 11. As for T-HCI, three isotopically substituted species are used to obtain this calculated geometry, but unlike T-HCl two very different geometries are found to fit the observed spectra equally well. These two structures are the only two that are consistent with the observed spectra. Structures with geometries intermediate to these are not consistent with the observed rotational contours. The indeterminacy in the structures results from the nearidentical placement of the center of mass of the water molecule with respect to the three molecular rotational axes of the complex as a whole, in both structures, for each of the three isotopically substituted species. The limit in accuracy of the determined rotational constants, imposed by the homogeneously broadened transitions, creates a situation in which further isotopic substitutions will not readily distinguish between these two structures. Microwave or infrared studies might again be useful in experimentally eliminating this ambiguity. Comparisons of these two T-H20 structures with that of T-HC1 clearly favor structure I. The N.-.H separation of 2.1 A of structure I is much closer to the observed T-HCl separation of 1.95 A than is the 2.4-A separation of structure 11. Calculations by DelBene4 on H 2 0 complexed to the diazines (pyrimidine, pyrazine, and pyridazine) indicate Ne. .H separations of about 2.1 A and slightly nonlinear N...H-O bonds of 1-5'. Neither experimental structure has a linear Ne H-0 interaction, but structure I has one of the water's hydrogens in a position reasonable for hydrogen bonding (ON...H = 64' compared to the similar angle in T-HCl of 46'), whereas structure I1 has both hydrogens further removed from a reasonable position for interaction with the nitrogen lone pair (ON...H = 85'). s - T e t r a ~ i n e ( H ~ 0The ) ~ .parameters used to fit the T-(H20)2 trimer are given in Table 111 and shown in Figure 5. The similarities between this structure and structure I of the T-H20 complex include a N. .H separation of 2.1 A for both complexes. The angle Bo in the T-(H20), complex is smaller than that of the T-H20 complex (37' vs. 71' for T-H,O for structure I).
-
( 4 ) DelBene, J. E. J . Am. Chem. SOC.1975,97,5330. Chem. Phys. 1976, 15, 463.
0
H'
L
I
H Figure 7. One possible geometric structure for T-(H20),.
However, both complexes maintain a nearly linear hydrogen bond, the deviation from nonlinearity being 10' and 8' for the T-(H20), and T-(H20), complexes, respectively. The fact that such a constrained structure best fits the observed spectra (yielding the most chemically reasonable distances) implies that a distorted water dimer interacting with the tetrazine ring is indeed an appropriate model for this complex. Several arguments support the structure of T-(H20)2 claimed as the most probable in Table 111. First, the interaction strength of H20 with H 2 0 is comparable to that of H 2 0 with tetrazine. Estimates of the (H20)2binding energy have come from a number of ab initio calculations5 and from some experimental determinations6 and are on the order of 4 to 5 kcal/mol (1400-1750 cm-I). We have recently studied7 the photochemistry of vibrationally excited T-H20 and, in a manner identical with that in which the s-tetrazine dimer binding energy was determined,* we were able to place a firm upper limit on the complex binding energy of 1490 cm-' and a probable lower limit of 790 cm-I. Thus, one should not expect a water dimer to remain undistorted in the presence of tetrazine. In addition, it should be noted that there is a free in-plane oxygen lone pair available for hydrogen bonding to a H on the tetrazine ring. Indeed, the proposed structure locates the oxygen corresponding to that free in-plane lone pair 2.2 A away from a s-tetrazine hydrogen (Figure 5). This might produce a cyclic stabilization of a six-membered ring composed of three chemical and three hydrogen bonds. In the T-H20 dimer the distance between the ring hydrogen atom and the oxygen atom is 2.05 A. This may mean that a stabilizing Hrlng-O interaction is present in the dimer as well as in the trimer. s - T e t r a ~ i n e ( H ~ 0The ) ~ .rotational constraints determined for the T-(H20)3 are consistent with a number of structures. Structures in which the three water molecules are symmetrically disposed around the tetrazine ring are chemically reasonable and only are discredited by the violation of the band-shift rule and analogy with the T-(H20), complex. One likely structure is shown in Figure I . Acknowledgment. This work has been supported by the National Science Foundation under Grant CHE-8311971 and by the donors of the Petroleum Research Fund, administered by the American Chemical Society. Registry No. Water, 7732-18-5; s-tetrazine, 290-96-0. Supplementary Material Available: Tables of frequencies and assignments of rotational lines for complexes of water with stetrazine (15 pages). Ordering information is given on any current masthead page. ( 5 ) (a) Matsuoka, 0.; Clementi, E.; Yoshimine, M. J . Chem. Phys. 1976, 64, 1351. (b) Hankins, D.; Moskowitz, J. W.; Stillinger, F. H. J. Chem. Phys. 1970, 53, 4544. (6) Curtis, L. A.;Frurip, D. J.; Blander, M . J. Chem. Phys. 1979,71,2703. ( 7 ) Morter, C.; Haynam, C. A.; Young, L.; Levy, D. H., to be submitted for publication. (8) Young, L.; Haynam, C. A.; Levy, D. H. J . Chem. Phys. 1983,79,1592.
