Metastable Isonitrosyl Structure of the Nitroprusside Anion Confirmed by Nuclear Inelastic Scattering Hauke Paulsen, † Ventzislav Rusanov, ‡ Ru ¨ diger Benda, † Christian Herta, † Volker Schu ¨ nemann, † Christoph Janiak, § Thomas Dorn, § Aleksandr I. Chumakov, ⊥ Heiner Winkler, † and Alfred X. Trautwein* ,† Contribution from the Institut fu¨ r Physik, Medizinische UniVersita¨t zu Lu¨beck, Ratzeburger Allee 160,D-23538 Lu¨beck, Germany, Department of Atomic Physics, UniVersity of Sofia, Faculty of Physics, 5 James Bourchier BlVd., BG-1126 Sofia, Bulgaria, Institut fu¨r Anorganische und Analytische Chemie, UniVersita¨ t Freiburg, Albertstrasse 21, D-79104 Freiburg, Germany, and European Synchrotron Radiation Facility, Boı ˆte Postale 220, F-38043 Grenoble, France Received May 22, 2001 Abstract: Nuclear inelastic scattering (NIS) measurements were performed on a guanidium nitroprusside ((CN 3 H 6 ) 2 [Fe(CN) 5 NO], GNP) monocrystal at 77 K after the sample was illuminated with blue light (450 nm) at50 K to populate the two metastable states, MS 1 and MS 2 , of the nitroprusside anion. A second measurement was performed at 77 K after warming up the illuminated crystal to 250 K where the metastable states decay to the groundstate. The measured spectra were compared with simulated NIS spectra that were calculated by using density functional methods.Comparison ofmeasured and simulated spectra provides strong evidence for the isonitrosyl structure of the metastable MS 1 state proposed by Carducci et al. (Carducci, M. D.; Pressprich, M. R.; Coppens, P. J. Am. Chem. Soc. 1997, 119 , 2669-2678). Introduction Sodium nitroprusside dihydrate (Na 2 [Fe(CN) 5 NO]‚2H 2 O, in short SNP‚2H 2 O) as well as many other nitroprusside species have become a promising basis for holographic information storage devices with extreme high capacity 1 since the existence of long-lived metastable states was discovered by Mo¨ssbauer spectroscopy in 1977. 2 In the following years extensive spec- troscopic studies have given further insight:there are two metastable states, MS 1 , which is stable up to 195 K, and MS 2 , which decays above 150 K. 3 Both metastable states can be populated reversibly from the ground state by illumination with lightat low temperature. The population is not performed directly but via transient states of higher energy. 4 There is no apparent dependency on the counterions or on the crystalline environment, which means that the metastability of the states must be a purely molecular phenomenon. 5 From the longevity of the metastable states Gu¨del 6 concluded that they have to be explained either by a structural change orby multielectron promotion. But despite all efforts the nature of the metastabl states remained in the dark for almost 20 years. In 1997, in their pioneering X-ray study, Carducci et al. 7 gave for the first time an explanation for the metastable states tha consistent with almostall the available experimental data. According to their explanation, MS 1 corresponds to an isoni- trosylgeometry of the nitroprusside (NP) anion and MS 2 corresponds to a side-on bonding of the nitrosyl group (Figur 1). Electronic structure calculations applying density function theory (DFT) by Delley et al. 8 yielded local minima on the energy hypersurface for the isonitrosyl structure and for the on structure. Laterstudies applying different DFT methods confirmed these results. 9,10 From the DFT calculations the † Medizinische Universita¨t zu Lu¨beck. ‡ University of Sofia. § Universita¨t Freiburg. ⊥ European Synchrotron Radiation Facility. (1) Woike,Th.; Krasser, W.; Bechthold, P. S.; Haussu¨ hl, S. Phys.ReV.Lett. 1984, 53,1767-1770. Woike,Th.;Kirchner, W.; Schetter, G.; Barthel, Th.; Kim, H.; Haussu¨hl, S. Opt.Commun. 1994, 106,6-10. (2) Hauser, U.; Oestreich, V.; Rohrweck, H. D. Z. Phys. 1977, A280, 17-25, 125-130; 1978, A284,9-19. (3) Zo¨ llner, H.; Woike, Th.; Krasser, W.; Haussu¨hl, S. Z. Kristallogr. 1989, 188,139-153. (4) Woike, Th.; Krasser, W.; Zo¨llner, H.; Kirchner, W.; Haussu¨hl, S. Z. Phys. D 1993, 25,351-356. (5) Zo¨llner, H.; Krasser, W.; Woike, Th.; Haussu¨hl, S. Chem. Phys. Lett. 1989, 161,497-501. (6) Gu¨del, H.U. Chem. Phys.Lett. 1990, 175, 262-266. (7) Carducci, M. D.; Pressprich, M. R.; Coppens, P. J. Am. Chem. Soc. 1997, 119,2669-2678. (8) Delley,B.; Schefer, J.; Woike,Th. J.Chem.Phys. 1997, 107,10067- 10074. (9) Blaha, P.; Schwarz, K.; Faber, W.; Luitz, J. Hyperfine Interact. 2000, 126 389-395. (10)Rusanov, V.; Gru¨nsteudel, H.; Paulsen, H.; Winkler, H.; Meyer-Klaucke, W.; Trautwein, A. X. Hyperfine Interact. 2000, 126, 159-162. Figure 1. Optimized geometry of (a) the ground state, (b) the side-on bonding (MS 2 ), and (c) the isonitrosyl (MS 1 ) structure of the NP anion as retrieved from BLYP/6-311+G(2d,p) DFT calculations. Published on Web 02/27/2002 10.1021/ja016239c CCC: $22.00 © 2002 American Chemical Society J. AM. CHEM. SOC. 9 VOL. 124, NO. 12, 2002 3007