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9317

J. Phys. Chem. 1993,97, 9317-9322

The Microwave Spectrum and Molecular Structure for Fe(N0)2(C0)2 S. C. Kukolich,' D. W. Wallace, D. M. Wickwire, S. M. Sickafoose, and M. A. Roehrig Department of Chemistry, The University of Arizona, Tucson, Arizona 85721 Received: June 18, 1993'

Microwave spectra for five isotopomers of iron dinitrosyl dicarbonyl were measured in the 4-16-GHz range using a Flygare-Balle type spectrometer. Numerous b-dipole transitions were observed for Fe(N0)2(C0)2, Fe15NONO(C0)2, Fe( 15NO)2(C0)2,Fe(15N0)213COC0,and 54Fe(15NO)2(C0)2. Measured rotational constants were analyzed to obtain an accurate gas-phase structure. Some of the structural parameters obtained are LN-Fe-N = 121(2)',LC-Fe-C = 101(2)',r(Fe-N) = 1.69(2)A,r(Fe-C) = 1.85(2)A,LFe-N-O= 176(2)O, and LFe-C-O = 177(2)". N M R and I R data are also given. Nitrogen quadrupole coupling tensors were obtained from hyperfine structure on transitions for Fe(14NO)2(C0)2 and Fe(14NO)(1sNO)(CO)z. The z principal axis for the quadrupole tensor is rotated 62' away from the F e N bond direction and principal axis components are eQq,, = -1.5 MHz, eQqxx = 1.0 M H z and e&,,,, = 0.53 MHz.

Introduction There has been much recent interest in complexes of NO since it was found to be a neurotransmitterl-3 and to play an important role in a wide variety of other physiological processes."8 The enzyme, guanylate cyclase, is activated by N O in the presence of heme.6.9 This activation is believed to take place through formation of a F e N O complex.10 It is then speculated' that the NO induced changes in coordination of the heme-iron cause guanylate cyclase to function as a signal transducer. Since NO is more easily reduced or oxidized than 0 2 or CO, many mechanisms could be used to control concentrations in biological systems. The kinetics of binding of NO to various hemoproteins were studied earlier." An iron carbonyl nitrosyl complex was reported in 1922 by Mond and Wallis,12 but the stated composition was not correct. The correct formula Fe(N0)2(C0)2 was given by Anderson13 with numerous references to the background work of W. Hieber. An electron diffraction study of Fe(N0)2(C02) was reported by Hedberg et a1.14 Precisevaluesfor the bond lengths were reported, but the uncertainties in bond angles were rather large. The structure of Fe(N0)2(C0)2 is shown in Figure 1. The C, symmetry with NO and CO groups in the symmetry planes considerably reduces the number of independent structure parameters and makes this a good molecule for microwave work. The fact that it is a near spherical top, combined with uncertainties in bond angles, caused difficulty in assigning the spectrum. Once it wasestablished that thedipoleaxis (C2sy"etryaxis) coincided with the b inertial axis, further work was fairly routine. Hyperfine structure splittings due to nitrogen quadrupole coupling were resolved and measured. The nitrogen quadrupole coupling tensor in this complex is quite unusual since it is rotated 61° away from the Fe-N and NO bond axes.

Experimental Section Iron dinitrosyl dicarbonyl was synthesized following the basic procedure given by King.15 The product was collected directly from the reaction mixture into a 77 K trap containing PzOs which was used to remove water when the collected product was warmed for transfer to a second trap. The 15N isotopomers were made using 15N sodium nitrite (Isotec 85-70009). Both a nearly pure 15N sample and a mixed 15NO-14N0 sample were synthesized to aid in assigning the spectra. The infrared spectrum for the normal isotopomer shows a strong, sharp doublet for NO stretching at 1768.0 and 1810.4 cm-1. Another strong, sharp *Abstract published in Advance ACS Abstracts. August 15, 1993.

--a

Figure 1. Structure of the Fe(N0)2(C0)2 complex. The complex has Cbsymmetry. The b principal inertial axis coincideswith the dipoleaxis and the Cz symmetry axis.

doublet for CO stretching was observed at 2035.2 and 2083.9 cm-l with the sample dissolved in parafin oil. NMR spectra were taken on a Brucker WM-250 spectrometer giving a single sharp 13Cresonanceat21 1.07 ppmincyclohexane-&and 210.22 ppm in benzene-d6. The linewidth was 7 Hz (fwhm). The 15N NMR spectrum of the 15N enriched sample gave a single sharp resonance at 31.68 ppm (relative to Nos- at 0 ppm) in cyclohexane-dl2. The microwave spectra were measured using a Flygare-Balle type spectrometer.16 First-run neon (Airco) was used as the expansion gas and passed through the sample tube maintained at -20 OC to provide an appropriate vapor pressure. The pulsed nozzle backing pressure was maintained at 0.54.8atm. The 0.5 atm pressure yielded a better S I N ratio at the higher transition frequencies, with 0.8 atm being better for f < 10 GHz. The best signal-to-noise ratios and simplest spectra were obtained with Fe( 15N0)2(C0)2since no 14Nhyperfine structure was present. The measured transitions for this isotopomer are listed in Table I. Forty rotational transitions were measured for this isotopomer. The signal-to-noise ratio was sufficiently high that 54Fe and 13C isotopomers could be seen in natural abundance for the nearly pure (95%) Fe(15N0)2(C0)2 sample. The 16 measured lines for 54Fe and 14 measured lines for 13C are listed in Table 11. The spectra for the Fe(N0)2(C0)2 and Fe(15NO)(NO)(CO)2 isotopomers had well-resolved splitting for transitions below 10 GHz. This hyperfine structure was due to 14N quadrupole

0022-365419312097-9317%04.00/0 0 1993 American Chemical Society

9318 The Journal of Physical Chemistry, Vol. 97, No. 37, 1993

TABLE I: Measured and Calculated Transition Frequencies for Fe(1SNOMCOM ,-, ,measd calcd measd-calcd J Kp KO J’ Ki K,’ (MHz) (MHz) (MHz) 1 1 2 2 2 2 2 2 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6

1 1 1 0 1 1 2 2 1 0 1 1 2 2 3 3 2 1 0 1 2 2 3 3 4 1 0 1 2 2 3 3 4 4 5 1 0 4 5 6

0 1 2 2 1 2 0 1 3 3 2 3 1 2 0 1 3 4 4 3 2 3 1 2 1 5 5 4 3 4 2 3 1 2 0 6 6 3 1

0

2 2 3 3 3 3 3 3 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7

2 2 0 1 2 2 3 3 0 1 2 2 3 3 4 4 1 0 1 2 3 3 4 4 5 0 1 2 3 3 4 4 5 5 6 0 1 5 6 7

1 0 3 3 2 1 1 0 4 4 3 2 2 1 1 0 4 5 5 4 3 2 2 1 0 6 6 5 4 3 3 2 2 1 1 7 7 2 2 1

4364.651 4379.543 6192.045 6279.625 6434.852 6483.004 6590.077 6592.100 8279.250 8340.703 8498.323 8602.055 8669.367 8679.561 8810.693 8810.873 10295.283 10362.413 10401.411 10555.594 10742.186 10772.365 10893.741 10895.000 11030.420 12441.225 12463.964 12607.903 12806.425 12874.300 12974.886 12979.940 13114.013 13114.130 13250.001 14516.569 14529.002 15197.739 15333.658 15469.528

4364.650 4379.545 6192.043 6279.624 6434.850 6483.005 6590.077 6592.099 8279.249 8340.703 8498.323 8602.057 8669.368 8679.560 8810.692 8810.871 10295.281 10362.416 10401.413 10555.595 10742.184 10772.366 10893.739 10895.004 11030.421 12441.225 12463.966 12607.901 12806.429 12874.301 12974.888 12979.939 13114.014 13114.133 13249.998 14516.567 14529.002 15197.740 15333.649 15469.533

0.0010 -0.0018 0.0021 0.0011 0.0021 40006 0.0004 0.0012 0.0008 0.0001 0.0004 -0.0017 -0.0007 0.0007 0.0014 0.0021 0.0019 -0.0027 -0.0016 -0.0007 0.0024 -0.0009 0.0019 -0.0035 -0.0005

-0.0004 -0.0017 0.0017 -0.0042 -0.0010 -0.0019 0.0011 -0.0008 -0.0035 0.0027 0.0023 0.0002 -0,0009 0.0091 -0.0052

Kukolich et al.

TABLE II: Results of Measurements and Calculations for the Isotopomers sFe(lSNO),(CO)z and Fe(1sNO)2(1jCO) (COP .-. measd measdmeasd measdJ K. KO J’ KO’ KO‘ P4Fe) calcd (13C) calcd I

.

I

~~~

2 2 2 2 2 2 3 3 3 3 3 3 3 4 4 4 4 4

1 0 1 1

2 2 1 0 1 2 2 3 3 1 0 1 2 4

2 2 1 2 0 1 3 3 2 1 2 0 1 4 4 3 2 1

3 3 3 3 3 3 4 4 4 4 4 4 4 5 5 5 5 5

0 1 2 2 3 3 0 1 2 3 3 4 4 0 1 2 3 5

3 3 2 1 1 0 4 4 3 2 1 1 0 5 5 4 3 0

6279.864 0.0007 6435.055 -0.0023 6590.248 6592.257 8279.350 8341.040 8498.633 8669.598 8679.706 8810.927 8811.100 10362.609 10401.826 10556.030 10742.509 11030.711

6175.037 6254.953 6401.345 6449.573 6547.742 6549.873 8254.600 8309.527

O.OOO8 0.0029 -0.OOO8 -0,0018 -0.0007 -0.0002 0.0029 0.0012

-0.0039 0.0003 -0.0015 0.0022 0.0002 0.0000 8619.930 -0.0040 0.0027 8630.664 0.0027 0.0051 8752.866 -0.0024 0.0012 8753.073 0.0051 0.0009 10329.979 -0.0029 O.OOO1 10363.975 -0.0015 0.0011 -0.0057 -0.0011

All frequencies are in megahertz and measd+alcd is the measured minus calculated (least-squaresfit) frequency. The standard deviations for the fits are 2.7 and 2.8 kHz.

iII

56Fe

54Fe

The calculated frequencies were obtained from a least-squaresfit to measured frequencies using parameters listed. The standard deviation for the fit is 2.7 kHz. a

coupling and measured transition frequenci_esar_egiyen in Tables V and VI. The total angular momentum, F = I J, is listed in the tables. A portion of the spectrum containing a line from Fe(15N0)2(C0)z and one from s4Fe(1sNO)z(C0)2is shown in Figure 2. The spectrometercavity was detuned toselectivelyattenuate the signal from the more abundant isotopomer.

+

Figure 2. Portion of the spectrum showing the 31423 transitions for 56Fe(15N0)2(C0)2 (left peak) and MFe(15N0)2(C0)2 (right peak). The frequencies are in megahertz relative to 8498.0 MHz. The cavity was detuned to selectivelyattenuate the S6Fesignal. This is an average of 216 beam pulse cycles.

Although the standard deviation for the fits is larger for these cases (1 1 kHz), the uncertainties in the rotational constants are not much larger than for the simpler l5N data.

Data Analysis and Rotational Constants The data for Fe(15NO)2(C0)2shown in Table I were fit using a Watson-type A-reduced Hamiltonian in the P representation” (seven parameters) and the best-fit parameter values are shown in column 1 of Table VI. Since the number of measured lines was much smaller for the 54Feand 13Cisotopomers, the distortion parameters for these fits were fixed at values obtained for Fe(15NO)2(CO)2(column 1, Table V). The A, B, and C rotational constants obtained for these fits are shown in columns 2 and 3 of Table V. The standard deviation for the fits was less than 3 kHz for the three isotopomers without 14N quadrupole coupling. The rotational and distortion constants for Fe(lSNO)(NO)(CO)2 and Fe(N0)2(C0)2are given in columns 4 and 5 of Table V. Since all transitions for these isotopomerswere more complex due to the 14Nhyperfine structure, a simpler rotational energy Hamiltonian involving only the DJ, DJK, and DK distortion constants (which have matrix elements similar to AJ, AJK,and A,) was used to reduce the total number of variable parameters.

Structure Determination The 15 measured rotational constants listed in Table V were used to determine the structure of iron dinotrosyl dicarbonyl. Two different procedures were used, but the resulting structures and uncertainties in coordinates were nearly the same in both cases. The first procedure was a direct least-squares fit using eight adjustable parameters to fit the rotational constants. This procedure resulted in a 12-kHz standard deviation for the fit and the adjustable parameters are the Cartesian coordinates for Fe, N, and C atoms and polar coordinates for the 0 atoms. The only problem with this procedure was a correlation between b-mordinatevalues for the various atoms, which increasedthe statistical uncertainty of thoseb coordinates. The resulting atom coordinates for this procedure (I) are given in Table VI. The second procedure (11) involved using the Kraitchman equationsl* to obtain coordinates directly for Fe, N, and C, followed by a least-squares fit to determine the oxygen atom

The Journal of Physical Chemistry, Vol. 97, No. 37, 1993 9319

Spectrum and Structure of Fe(N0)2(C0)2

TABLE III: Results of Measurements and Least-Squares Fit Calculations for Fe(15NO)(NO)(CO)zTransitions. measd

calcd

measdcalcd

8705.913 10421.688 10463.728 10463.744 10609.948 10784.028 10784.085 10807.286 10925.807 10925.872 10926.681 10926.735 11055.871 11055.912 12513.494 12539.220 12677.332 12677.363 12862.970 12863.026 12915.695 13018.546 13150.345 13150.426 13279.929 14602.041 14602.081 14616.830 15374.393 15374.460 15503.885 15503.920

8705.917 10421.700 10463.734 10463.754 10609.950 10784.036 10784.081 10807.295 10925.814 10925.871 10926.688 10926.747 11055.866 11055.894 12513.506 12539.228 12677.314 12677.330 12862.977 12863.001 12915.679 13018.553 13150.350 13150.425 13279.912 14602.058 14602.066 14616.835 15374.415 15374.450 15503.902 15503.917

-0.004 -0.012 -0.006 -0.010 -0.002 -0.008 0.004 -0.009 -0.007 0.001 -0.007 -0.012 0.005 0.018 -0.012 -0.008 0.018 0.033 -0.007 0.025 0.016 -0.007 -0.005 0,001 0.017 -0.017 0.015 -0,005 -0.022 0.010 -0.017 0.003

measd-

J

Kp KO F J’ KP) K,’ F‘

1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3

1 1 1 1 1 1 0 1 1 1 1 1 1 2 2 2 2 2 2 2 1 1 0 0 1 1 1 2 2 2 2 2

0 0 1 1 2 2 2 1 1 1 2 2 2 0 0 1 1 1 2 2 3 3 3 3 2 3 3 1 1 1 2 2

2 1 0 2 2 3 3 1 3 2 1 3 2 3 2 3 1 2 3 4 3 4 3 4 4 2 4 2 4 3 2 4

2 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 4 4 4

2 2 2 2 0 0 1 2 2 2 2 2 2 3 3 3 3 3 1 1 0 0 1 1 2 2 2 3 3 3 3 3

1 1 0 0 3 3 3 2 2 2 1 1 1 1 1 0 0 0 3 3 4 4 4 4 3 2 2 2 2 2 1 1

3 2 1 3 3 4 4 2 4 3 2 4 3 4 3 4 2 3 4 5 4 5 4 5 4 3 5 3 5 4 3 5

measd

calcd

calcd

J

Kp

KO F J’ KP) K,,’ F‘

4377.730 4377.786 4390.255 4390.318 6227.395 6227.460 6314.690 6460.738 6460.779 6460.812 6501.181 6501.273 6501.468 6606.863 6606.917 6608.406 6608.440 6608.475 8233.520 8233.658 8326.108 8326.146 8389.559 8389.587 8538.057 8624.978 8625.039 8697.994 8698.013 8698.073 8705.793 8705.834

4377.729 4377.790 4390.260 4390.314 6227.380 6227.456 6314.699 6460.746 6460.774 6460.824 6501.174 6501.273 6501.457 6606.859 6606.918 6608.406 6608.427 6608.473 8233.514 8233.655 8326.110 8326.141 8389.565 8389.584 8538.057 8624.978 8625.043 8698.000 8698.015 8698.082 8705.807 8705.829

0.001 -0.004 -0.005 0.004 0.015 0.004 -0.009 -0.008 0.005 -0.012 0.007 0.000 0.011 0.004 -0.001 0.000 0.013 0.002 0.006 0.003 -0.002 0.005 -0.006 0.003 0.000 0.000 -0.004 -0.006 -0,002 -0.009 -0.014 0.005

3 4 4 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6

2 1 0 0 1 2 2 2 3 3 3 3 4 4 1 0 1 1 2 2 2 3 4 4 5 1 1 0 5 5 6 6

2 4 4 4 3 2 2 3 1 1 2 2 0 0 5 5 4 4 3 3 4 2 1 2 0 6 6 6 1 1 0 0

3 5 4 5 5

5 4 5 5 4 5 4 5 4 6 6 5 6 6 5 6 6 6 6 6 6 7 7 7 6 7 6

4 5 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7

3 0 1 1 2 3 3 3 4 4 4 4 5 5 0 1 2 2 3 3 3 4 5 5 6 0 0 1 6 6 7 7

1 5 5 5 4 3 3 2 2 2 1 1 1 1 6 6 5 5 4 4 3 3 2 1 1 7 7 7 2 2 1 1

4 6 5 6 6 6 5 6 6 5 6 5 6 5 7 7 6 7 7 6 7 7 7 7 7 7 8 8 8 7 8 7

a Frequencies in MHz, with quadrupole coupling parameters eQq, = -0.67(2) MHz and eQqbb = 0.16(2) MHz. The best-fityalu_esfo_r rotational and distortion constants are given in Table V. The standard deviation for the fit is 11 kHz. The total angular momentum is F = I J .

+

coordinates. In general, this would be expected to be a more accurate procedure, but for this molecule results are estimated to be slightly less accurate than the direct fit (procedure I). The atom coordinates obtained from the Kraitchman analysis are given in Table VII. The interatomic distances and interbond angles obtained from both procedures are given in Table VIII. We have also included some average bond lengths from the Cambridge Structural Data Baselg for comparison. We note that the Fe-C-0 and F e N - 0 bond angles both appear to be different from 180°, although the difference is not much greater than experimental and model uncertainties. The F e C - 0 bonds appear to be bent in a direction to move the 0 atoms further apart, whereas the Fe-N-0 angles appear to be bent in a direction to move 0 atoms closer together. This last result is consistent with the N O dimer structure:O where the N-N-0 bond angles are very near loo’, indicating a possible attractive 0-0interaction for the N O groups. The bending, or ‘geniculation” of the Fe-N-0 bonds to bring the 0 atoms closer together in dinitrosyls has been observed in other compounds2’and discussed in terms of the orbitals involved by Enemark and Feltham.22 The bond length and angles from the electron diffraction study are also included in Table VII, and we note that the bond lengths are in quite good agreement with the present results. The result that the electron diffraction values for N O and CO bond lengths are closer to their average value than for the microwave data is not surprising since the peak for these two distances was not resolved in the radial distribution plot.

Nitrogen Quadrupole Coupling Most transitions for the Fe(lSNO)(NO)(CO)zand Fe(NO),(C0)z isotopomers have resolved splittings due to nitrogen quadrupole coupling. These splittings are analyzed by including terms in the Hamiltonian involving the nitrogen spins ZI and 1 2

and rotational angular momentum J as discussed earlier.23 The additional parameters are the quadrupole couplingstrengths eQqw and eQqbb along the principal inertial axes of the molecule. The measured and calculated transition frequencies for Fe(lSNO)(NO)(C0)2 are given in Table 111, along with the “best-fit” quadrupole coupling strengths. The hyperfine structure for the normal isotopomer is a little more complex than for the 15N-14N isotopomer since the quadrupole coupling for two equivalent nitrogen atoms must be considered. The basic matrix elements23can still be used. but we A

d

+

d

must also consider the total nitrogen spin Z = Z, I,. Since the nitrogen atoms are bosons, the “even” parity spin states Z = 2 and Z = 0 must occur for rotational states which are even functions of a ?r rotation about the b axis and the “odd” state I = 1 will occur for rotational states which are odd functions with respect to ?r rotation about the b axis. In terms of the usual Kp and KO rotational state quantum numbers, the “even” states will have P(Kp).P(Ko) = odd, where P ( K ) denotes the parity of the K quantum number. This reduces the number of possibletransitions somewhat, but the hyperfine structure was still unresolved for the higher J transitions. The results of measurements and leastsquares fit calculations for the normal isotopomer are listed in Table IV. The principal axis orientation and principal axis values for the nitrogen quadrupole coupling tensor can be obtained from data on the two isotopomers containing 14N since the inertial axis system for Fe(’SNO)(l4NO)(C0)2 is rotated by 3.03’ about the c axis (which coincides with they axis in Figure 3) relative to the Fe(l4NO)(CO)z axes. Since this angle is small, however, the uncertainty in the componentswill be fairly large. The quadrupole splittings in this “first order” quadrupolecoupling situation depend only on the diagonal elements eQqw, eQqbb, and eQqccin the principal axis system and only diagonal elements are nonzero in the x, y, z principal axis system for the quadrupole coupling

9320

Kukolich et al.

The Journal of Physical Chemistry, Vol. 97,No.37, 1993

TABLE I V Results of Measurements and Least-Squares Fit Calculations for Fe(NO),(C0)2 Transitions. I J K, K, F J' K,' K,' F' measd calcd measd-calcd I J K, K, F J' K,' KO' F' measd 1 1 1 1 1 1 2 2 0 1 1 2 0 2 2 0 2 2 1 1 1 2 0 2 2 1 1 1 2 1 1 2 2 2 2 2 1 1 1 1 1 0

1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3

0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 2 2 2 2 2 2 2 3 2 2 2 2 1 1 1 0 0 1 1 1 1

1 1 1 0 0 0 1 1 1 2 2 2 2 2 2 1 1 1 2 2 2 0 0 0 0 1 1 1 1 1 1 2 2 3 3 3 3 3 2 2 2 3

1 1 2 1 2 1 3 2 1 3 2 0 2 4 3 2 4 3 2 3 1 3 2 4 3 2 1 3 5 2 4 4 5 3 4 5 4 2 3 4 2 3

2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 4 4 4

1 1 1 2 2 2 2 2 2 0 0 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3 3 2 1 1 1 1 0 0 0 1 1 2 2 2 2

2 2 2 1 1 1 0 0 0 3 3 3 3 3 3 2 2 2 1 1 1 1 1 1 1 0 0 0 2 4 4 3 3 4 4 4 4 4 3 3 3 2

1 2 3 2 3 1 4 3 2 4 3 1 3 5 4 3 5 4 3 4 2 3 2 5 4 3 2 4 6 3 5 5 6 4 5 6 5 3 4 5 3 4

4257.628 4257.887 4257.950 4389.871 4389.962 4390.266 4400.534 4400.764 4400.889 6264.257 6264.357 6349.478 6349.535 6349.587 6349.633 6485.811 6485.856 6485.969 6519.941 6520.126 6520.226 6621.698 6621.756 6622.159 6622.221 6623.355 6623.392 6623.434 8130.597 8182.374 8182.448 8282.995 8283.217 8374.394 8374.434 8374.493 8438.319 8438.354 8576.891 8576.931 8576.974 8649.940

4257.621 4257.886 4257.954 4389.872 4389.969 4390.270 4400.531 4400.758 4400.891 6264.246 6264.352 6349.479 6349.534 6349.597 6349.638 6485.813 6485.862 6485.976 6519.938 6520.119 6520.217 6621.696 6621.760 6622.160 6622.230 6623.358 6623.410 6623.436 8130.593 8182.382 8182.458 8282.973 8283.201 8374.389 8374.445 8374.487 8438.331 8438.355 8576.899 8576.945 8576.972 8649.937

0.007 0.001 -0.004 -0.002 -0.007 -0.004 0.003 0.007 -0.002 0.011 0.005 -0.001 0.001 4.010 -0.005 -0.002 -0.006 -0.007 0.003 0.007 0.009 0.002 -0.004 -0.001 -0.009 -0.003 -0.018 -0.002 0.004 -0.008 -0.010 0.022 0.016 0.005 -0.011 0.006 -0.012 -0.001 -0.008 -0.014 0.002 0.003

1 1 2 2 2 1 2 1 1 2 2 2 1 2 2 1 2 2 2 2 2 1 1 1 1 1 2 0 2 2 1 1 2 2 2 2 2 1 2 1 2 2

3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6

2 2 2 2 3 3 3 2 1 0 1 1 1 2 3 3 4 4 2 2 2 1 0 1 2 3 3 4 4 5 1 1 0 1 2 2 3 3 4 4 5 6

1 1 2 2 0 1 0 3 4 4 3 3 4 2 1 2 0 1 4 4 4 5 5 4 3 2 3 1 1 1 6 6 6 5 4 4 3 4 3 2 1 0

3 4 5 4 5 4 2 5 3 6 6 5 5 6 6 4 6 4 5 4 7 7 5 6 6 6 7 5 7 7 7 6 8 8 8 7 8

6 8 7 8 7

4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7

3 3 3 3 4 4 4 1 0 1 2 2 2 3 4 4 5 5 1 1 1 0 1 2 3 4 4 5 5 6 0 0 1 2 3 3 4 4 5 5 6 7

2 2 1 1 1 0 1 4 5 5 4 4 3 3 2 1 1 0 5 5 5 6 6 5 4 3 2 2 2 0 7 7 7 6 5 5 4 3 2 3 2 1

4 5 6 5 6 5 3 6 4 7 7 6 6 7 7 5 7 5 6 5 8 8 6 7 7 7 8 6 8 8 8 7 8 9 9 8 9 7 9 8 9 8

8725.290 8725.389 8731.357 8731.468 8850.745 8850.813 8850.857 10407.576 10482.256 10526.182 10663.285 10663.346 10793.052 10824.373 10956.125 10956.741 11078.629 11078.715 12532.956 12533.024 12533.057 12586.883 12614.867 12745.679 12917.685 13060.673 13063.061 13184.362 13184.395 13306.474 14688.563 14688.594 14705.341 14825.183 15004.319 15004.347 15162.908 15170.285 15289.779 15290.073 15412.165 15534.293

calcd

measd-calcd

8725.292 8725.379 8731.355 8731.474 8850.758 8850.818 8850.855 10407.582 10482.254 10526.179 10663.305 10663.336 10793.060 10824.378 10956.128 10956.734 11078.623 11078.691 12532.959 12533.023 12533.073 12586.902 12614.868 12745.667 12917.672 13060.646 13063.057 13184.336 13184.384 13306.470 14688.572 14688.582 14705.340 14825.156 15004.316 15004.352 15162.929 15170.269 15289.802 15290.090 15412.185 15534.276

-0.002 0.010 0.002 -0.006 -0.013 -0.005 0.002 -0.005 0.002 0.003 -0.020 0.010 -0.008 -0.005 -0.003 0.007 0.006 0.024 -0.003 0.000 -0.016 -0.019 -0.001 0.01 1 0.013 0.027 0.005 0.026 0.01 1 0.004 -0.009 0.012 0.001 0.026 0.003 -0.005 -0.020 0.016 -0.023 -0.018 -0.020 0.018

Frequencies in MHz with quadrupole coupling parameters eQq, = -0.179(3) MHz and eQqy = _0.27(,3)MHz. The best-fit values foi. rotational and distortion constants are given in Table V. The standard deviation for the fit is 11.5 MIz. I = I1 + I2 is the total nitrogen spin.

TABLE V Rotational and Centrifugal Distortion Constants Obtained from Least-Squares Fits to Measured Rotational Transitions for Five Isoto~)mers* isotopomer Fe(l5NO)2(C0)2 54Fe(15NO)2(C0)2 Fe(l5N0)2(l3C0)(CO) Fe(15NO)(NO)(C0)2 Fe(N0)2(C0)2 ~~~

A (MHz) E (MHz) C (MHz) AJ ( k H 4 AJK ( k H 4 AK ( k H 4 8, ( k H 4 8~ ( k H 4 gtit W z )

1109.8500(4) 1048.1508(8) 1035.1075(6) 0.28(1) -0.05(4) 0.1O(3) 0.051 (6) 0.9( 1) 2.7

1109.8815(3) 1048.1360(9) 1035.1452(3)

1102.0828(4) 1044.6382(8) 1031.7068(3)

F F F F F

F F F F F

2.7

2.8

1112.0782(14) 1052.6983(11) 1041.5267(12) DJ = 0.19(2)b DJK = 0.26(6)b D~=-0.11(6)~

1113.9861(11) 1057.5566(8) 1047.9913(9) 0.20(l)b 0.30(5)b -0).13(4)b

11.

11.

Listed errors are 28. F listings indicate value was fixed at the parameter value obtained for the Fe(15N0)2(C0)2fit. b The three-parameter centrifugal distortion basis, DJ, DJK, and DK,was used for these isotopomers. tensor. With these simplifications, we find, for a rotation angle 0 about the c inertial axis eQq,( 1 - 2 COSz 8) = eQqb6

+ eQqcc(cosZ0)

We h o w one principal axis component already (eQqyy= eQqcc) since the c axis is perpendicular to a mirror plane for the molecule. Using the above formula we can determine an angle 0 for the normal isotopomer and 0' = 0 + 3.03' for the Fe(15NO)(NO)(C0)z isotopomer which will give the same value for eQqrr, the largest principal axis component for bothisotopomers. The results of this analysis are shown in Table IX. The z principal axis of the quadrupole coupling tensor intersects the Fe-N bond direction at an angle of 62O as shown in Figure 3.

Quadrupole coupling'data for the NO ligand can be divided into two groups, linear and symmetric top molecules with cylindrical symmetry for the electric field gradient tensor, and asymmetric top molecules with different values for the three principal axis electric field gradient components. Molecules with cylindrically symmetric quadrupole coupling tensors are NO with eQqzz = -1.86 cyclopentadienyl nickel nitrosyl with eQqzz = -1.22 MHz25 and CO(CO)JNOwith eQqzz = -1.6 MHz.26 These values, when interpreted in terms of the Townes-Dailey theory (see discussion ref 25) have a slightly higher 2p, atomic orbital population than 2p, or 2py orbital populations for the nitrogen 2p atomic orbitals. The eQqxxand eQqyycomponents will be small, positive and equal to -'/2eQqZz for these cases.

The Journal of Physical Chemistry, Vol. 97, No. 37, 1993 9321

Spectrum and Structure of Fe(NO)*(C0)2

TABLE VI: Cartesian Coordinates (A)for the Atoms Obtained Using Procedure I, a Direct Fit to the Observed Rotational Constants' atom

U

c1 c2 01 02 N1 N2 03 04

0.000 0.000 0.000 0.000 1.469 -1.469 2.455 -2.455 0.000

Fe a

b

C

1.173 1.173 1.841 1.841 -0.831 -0.831 -1.477 -1.477

1.427 -1.427 2.344 -2.344

0.000

0.000

b

t

0.OOO 0.000 0.000 0.000

Estimated uncertainties in the u and c coordinates are 0.01 and 0.02

A in the b coordinates.

TABLE M. Coordinates of Atoms (A) Determined Using the Kraitchman Method and Appropriate Combinations of Measured Rotational Constants' atom

a

b

C

c1 c2 N1 N2 Fe

0.000 0.000 1.466 -1.466 0.000

1.085 1.085 -0.25 -0.925 -0.088

1.428 -1.428 0.000 0.000 0.000

'The coordinates are relative to the center of mass of the normal isotopomer.

TABLE VIE Bond Lengths and Interbond Angles Obtained with the Fit Procedure I and Modified Fit Procedure II, Values from Ref 14, and Average Bond Lengths from the Cambridge Structural Data Base (CSD; Ref 19)' procedure I

procedure I1

c1-01 c2-02 Cl-Fe C2-Fe N 1-Fe N2-Fe N1-03 N2-04

1.134 1.134 1.848 1.848 1.688 1.688 1.179 1.179

Cl-Fe-CZ Nl-Fe-N2 Fe-Cl-01 Fe-CZ-02 Fe-N1-03 Fe-N2-04 01-FeO2 03-Fe-04

101.2 121.0 176.7 176.7 176.3 176.3 103.7 117.9

Bond Lengths (A) 1.128 1.128 1.848 1.848 1.689 1.689 1.182 1.182 Angles (deg) 101.2 120.5 176.3 176.3 176.6 176.6 104.0 117.7

ref 14

CSD (av)

1.140 1.140 1.883 1.883 1.688 1.688 1.167 1.167

1.18(8) 1.18(8) 1.80(7) 1.80(7) 1.67(6) 1.67(6) 1.17(4) 1.17(4)

110 114 180 180 180 180 110 114

a Fe-C and C-0 bond lengths (CSD) were averaged over 4700 entries and F-N and N-0 bond lengths over 240 entries. Estimated uncertainties of bond lengths are h0.02 A and angles *2O.

The quadrupole coupling components for asymmetric top molecules may have large deviations from cylindrical symmetry. Examples are NO dimer20 with eQq,, = -4.066, eQqbb = -2.242, and eQqcc = 6.037 MHz and CH3NO with eQq,, = 0.5, eQqbb = -6.02, and eQqcc= 5.52 MHz.2' Both of these molecules have very nonlinear X-N-O bonding with N-N-O and C-N-0 interbond angles of 99.6' and 112O. The large positive value for eQqcc (the direction perpendicular to the X-N-0 plane) is presumably due to a significantly lower orbital population for electrons occupying the nitrogen 2p orbital perpendicular to the X-N-O plane. The present results for Fe(N0)2(C0)2fall between the above "linear" and "strongly bent" X-N-O bonding cases. The z principal axis component eQqzz= -1.5 MHz is close to the above values for NO, CpNiNO, and Co(C0)3NO z-axis components. The eQqxxand eQqyvcomponents do not differ from each other nearly as much as corresponding values for (N0)z or CH3NO.

X Figure 3. Fe(N0)2(C0)2 complex with the u and b inertial axes, and the N O groups in the plane of the page. The x and z axes are the principal axes for the nitrogen quadrupole coupling tensor. The z quadrupole tensor principal axis makes an angle of 62' with the Fe-N bond axis. The components of the nitrogen quadrupole coupling tensor are eQqzz -1.5 MHz, and eQqxx = 1.0 MHz.

TABLE M: Quadrupole Coupling Tensor Components for the Normal Isotopomer (I), the Fe(lSNO)(NO)(CO)* Isotopomer (II), and Principal Axis Components (Values in

pridcip1:axis'components

eQq, = -1.5(2)

eQqu = 1.0(2)

eQq, = 0.51(4)

The value for eQqyy = 0.51 MHz, which is the component perpendicular to the X-N-O plane, is less positive than eQqxx rather than more positive as was seen for the "strongly bent" cases. This could be due to orbital contributions from iron d orbitals which were not present in the above "strongly bent" cases. The 15N NMR line at 31.7 ppm is in the region expected for linear or nearly linear NO groups.28

Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. Lucio Flores and Christy Springfield recorded and analyzed some of the data. David Wigley provided essential advice on synthesis. Bob Feltham and John Enemark provided helpful advice on data interpretation and references. The pulsed beam microwave spectrometer was constructed with funds from the NSF. References and Notes (1) Bredt, D. S.;Snyder, S . H. Neuron, 1992,8, 8-11. (2) Barinaga, M. Science, 1991, 254, 1296-1297. (3) Nowak, R.J. NZH Res. 1992,4,49-55. (4) Moncada, S.;Palmer, R.M.J.; Higgs, E. A. Pharmacol. Reu. 1991, 43, 109-141. (5) Ignarro, L. J. Circ. Res. 1989, 65, 1-21. (6) Ignarro, L. J. Semin. Hematol. 1989, 26, 63-76. (7) Traylor, T. G.; Sharma, V. S . Biochemistry 1992, 31, 2847-2849. (8) Ignarro, L. J. NZH Res. 1992, 4, 59-62. (9) Ignarro, L. J.; Adams, J. B.; Horwitz, P.M.; Wood, K. S.J. Biol. Chem. 1986, 262,4997-5002. (10) Traylor,T. G.;Duprat,A. F.;Sharma,V. S . J. Am. Chem.Soc. 1993, 219,810-811. (11) Rose, E. J.; Hoffman, B. M. J. Am. Chem. Soc. 1983, 205, 28662873. (12) Mond, R. L.; Wallis, A. E. J. Chem. Soc. 1922, 122, 32-35. (13) Anderson, J. S . 2.Anorg. Allg Chem. 1932,208, 238-248. (14) Hedberg, L.; Hedberg, K.; Satija, S.K.;Swanson, B.I. Inorg. Chem. 1985, 24, 2766-2771. (15) King, R.B. OrganomerallicSynrheses;Eisch, J. J., King, R. B., Eds.; Academic Press: New York, 1965; Vol. 1, pp 167-168. See also: Hieber, W.; Beutner, H. 2.Anorg. Allg Chem. 1963,320, 101-111. (16) Balk, T. J.; Flygare, W. H. Rev. Sci. Znstrum. 1981, 52, 33.

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