Gas-phase generation and characterization of aminosilyliumylidene

Mikhail N. Glukhovtsev, Addy Pross, H. Bernhard Schlegel, Robert D. Bach, and Leo Radom. Journal of ... Norman Goldberg and Helmut Schwarz. Accounts o...
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J. Phys. Chem. 1993,97, 10687-10693

10687

Gas-Phase Generation and Characterization of SiNH2+ and SiNHi: A Combined Neutralization-Reionization Mass Spectrometry and ab Initio Molecular Orbital Study? Norman Goldberg, Jan HruUk,* Muhammad Iraqi, and Helmut Schwarz* Institute of Organic Chemistry, Technical University of Berlin, Strasse des 17. Juni 135, D - 10623 Berlin, Germany Received: July 19, 1993"

Mass spectrometric studies strongly suggest the existence of the long-sought-after SiNH2+('Al) ion in the gas phase; this species can be successfully neutralized to SiNH2'(2A1), which is thought to be of astrochemical interest. The experimental findings are complemented by extensive ab initio MO calculations, employing Pople's G1 and G 2 programs; in addition, relevant parts of potential energy surfaces belonging to different electronic states are calculated.

Over the past years elusive silicon-containing molecules have received a great deal of interest not least because of their astrochemical and astrophysical importance, and many examples have been observed or predicted to exist in interstellar and circumstellar These species often undergo ion-molecule reactions resulting in the formation of new classes of compounds. Research in this field is also motivated by the fact that Si*+ is abundant in the outer part of the earth's stratosphere and is believed to play a significant role in the chemical processes taking place in this region. The interest in silicon chemistry is further spurred by the latest developments in microelectronics where, during the plasma etching and in chemical vapor decomposition processes, silicon is involved in a number of gas-phase reactions. For all these reasons silicon-bearing molecules have formed the subject of numerous experimental and theoretical studies, with special emphasis on the understanding of chemical reaction pathways from atomic silicon cations Si'+(2P)to neutral siliconcontaining molecules.*-I1 Matrix-isolation studies in conjunction with infrared and microwave spectroscopy have helped to characterize small silicon-containing molecules. Mass spectrometric experiments using guided ion beam, Fourier transform mass spectrometry and neutralization-reionization techniques have also provided insight into the structures and stabilities of such species; additional information about their bimolecular reactivity is obtained from flowing afterglow experiments. From the very beginning, quantum chemical methods have been used as valuable tools to support, interpret, guide, and sometimes even predict the experimental findings. For example, many efforts have been made to characterize Si-X multiple bonded systems in order to understand the differences in the chemical behavior of silicon as compared with its lower homologue carbon.12 Among the SiNH, species, in an early studyl3 silaisonitrile (SiNH) was reported to be a stable molecule in an argon matrix. This finding has been supported recently by Maier et al.l4 Upon photolysis of SiH3N3 at 4 K, in addition to the observation of SiNH, they were able to generate aminosilylene (HSiNH2). The microwave spectrum of the SiNH moleculelSL7has been measured as well as calculated by ab initio MO calculationsL6and found to be in agreement with the findings of the matrix experiments, thus demonstrating that the system has a SiNH connectivity. Recently, in a further study19 we were able to characterize the SiNH*+ion, as well as its neutral form, by a combined ion-beam and ab initio MO study. No experimental data, however, dealing with the structures of the [H2SiN]+ isomers and their neutral t Dedicated to Dr. V.H a n d , Prague, on the occasion of his 70th birthday.

* Author to whom correspondence should be addressed.

t On leave from: Institute of Macromolecular Chemistry, Czech Academy of Sciences, Heyrovsky Sq. 2, 16206 Praha, Czech Republic. Abstract published in Aduance ACS Abstracts. September 15, 1993.

0022-365419312097-10687$04.00/0

counterparts are so far available. It has been suggested by Bohme20 that the reaction process depicted in eq 1 Si'+(2P)

+ NH,

-

+

-

SiNH:

+ H'

(1) is of prime importance for the formation of Si-N bonds in the chemistry of interstellar clouds in which SiNH2+is believed to be neutralized by dissociative electron ion recombination (eq 2) to generate SiNH (and not HSiN which has an energy at least 56 kcal/mol higher than SiNHl9) SiNH2+ e-

SiNH

+ H'

(2) Based on bracketing experiments, the proton affinity (PA) of the nitrogen site of SiNH was determined to PA = 203 f 2 kcal/mol.20 However, the difficulties often associated with the unambiguous detection and structural characterization of these molecules and the evaluation of mechanistic details pertinent to their genesis in interstellar clouds point to the importance of laboratory experiments. Here one faces, however, a fundamental obstacle in that neutral products of ion-molecule reactions usually escape experimental detection. Nevertheless, by using the technique of neutralization-reionization mass spectrometry (NRMS),Z1-2*we were recently able to generate and structurally characterize numerous elusive silicon-containing molecules of interstellar interest.19.29-35 As mentioned, small silicon-containing systems have formed the subject of extensive ab initio MO calculations in order to get information about the nature of the silicon bonds.36 With regard to the present work, single-pint calculationsat the MP4(SDTQ)63 lG* level have been used to calculate substituent effects on the stabilization energies in Six+, HSiX*+, and H2SiX+ (X = H, CH3, NH2)." For the ion SiNH*+ it has been concluded that this system gains the largest stabilization of all systems calculated. Flores and Largo-Calverizo3* reported the existence of three isomers on the potential energy surface (PES) of [H2SiN]+ with the SiNH2+ species ( C b ) corresponding to the most stable one. According to MP4/6-31G*//HF/4-31 calculations, the energy difference between SiNH2+ and HNSiH+ amounts to 58.1 kcal/ mol.39 An approximate characterization of the transition structure for the silicon to nitrogen hydrogen atom migration was obtained by varying the NSiH angle without a full geometry optimization, and a value of 21.4 kcallmol was calculated for the transition structure with respect to HNSiH+. The estimated proton affinity for SiNH, to generate SiNH2+, converges to a value of ca. 200 kcal/mol, in good agreement with Bohme's bracketing experiments (PA = 203 f 2 kcal/mol).20 Furthermore, MP4/6-31G*// HF/6-3 1G calculation^^^ predict the triplet-state S ~ N H ~ + ( ' A I ) to be more stable by 26 kcal/mol than the corresponding singlet 0 1993 American Chemical Society

10688 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993

structure of SiNH2+('Al). As can be concluded from the series of publications by Flores et al.384 as well as the more recent work by Melius and Ho,41 the S C F procedure does not provide reliable structural and energetic information about the [HzSiN]+ potential energy surface and also fails to properly characterize stationary points. In this paper we will report the results of a detailed theoretical and experimental investigation of various isomeric structures of [H2SiN]+ cations and their neutral counterparts. In addition, experiments were performed aimed at generating one or the other of the ionic and neutral species, and the connectivity of one isomer has beenverified by collisional activation (CA) and neutralizationreionization (NR) experiments. The ab initio M O calculations were performed in the framework of the GAUSSIAN 1 and 2 programs.424 In addition to the minima of [H2SiN]+ described earlier,37s39*41 we also investigated the transition structures (TSs) for the unimolecular rearrangements and reinvestigated some of the different electronic states of the [H2SiN]+isomers. To obtain consistent arguments for the interpretation of the CA and N R mass spectra, we also examined the potential energy surface of the neutral species and all related fragmentation channels at the same level of theory. Experimental Section All collision experiments were performed on a modified largescale four sector tandem mass spectrometer of BEBE configuration (B stands for magnetic and E for electric sector). The details of the machine and its operation have been described The [H2SiN]+ ions were produced by electron bombardment (electron energy 50-100 eV) of a mixture of iodosilane (SiH3I) and ammonia (NH3) in a CI source (source conditions: repeller voltage ca. 0 V; ion source temperature 200 "C; acceleration voltage 8 kV). The ions ( m / z 44) of interest were mass selected by means of B(l)E(l) a t a resolution of m / A m = 2500 (10% valley definition). In the CA experiments a beam of mass-selected [HzSiN]+ ions, having 8-kV translational energy, was collided with He (80% transmission, T) in the second cell of the differentially pumped tandem collision cells. Possible interference at m / z 44 from unresolved isobaric ions (28SiN+,29SiNH'+) is minor due to the dominance of the [HzSiN]+signal in that region of the chemical ionization mass spectrum. In the N R M S experiments the 8-kV beam of B( 1)E(1) mass-selected [H2SiN]+ ions was neutralized by colliding it with xenon (80% T). Unreacted ions were deflected from the beam of neutrals by applying a voltage on a deflector electrode situated between the two collision cells. Subsequent reionization of the beam of neutral [HzSiN]' was brought about in the second cell by collision with oxygen (80% T). In both experiments the mass spectra of the resulting fragment ions were recorded by scanning B(2). The minimal lifetime t (identical with the transit time for the neutrals from the first to the second collison cell) is >4 ps. To improve the signal-to-noise ratio, signal averaging techniques were used by accumulating 50-80 scans and on line processing the data with the AMD-Intectra data system. Computational Details The detailed description of the main part of the method has formed the subject of an earlier p ~ b l i c a t i o nand, ~ ~ therefore, will not be repeated here in detail. The procedure is quite similar to Pople's GAUSSIAN- 1 (Gl) and GAUSSIAN-2 (G2) approach.@ The main differences are as follows: (i) We use the larger 6-31G**4savalencebasisset (insteadof6-31G* asin thestandard case of G144) for the MP2(FULL) geometry optimization (including all electrons in the correlation calculation); (ii) the same level of theory (Le., MP2(FULL)/6-31G**) has been employed for the calculation of vibrational frequencies; (iii) to obtain the correction AE(ZPE) for G 1, we have scaled the ZPVEs by a factor of 0.893.44

Goldberg et al. At the MP2/6-31G** calculated geometry we performed a series of single-point calculations in the Moller-PlessetM framework up to the fourth order including all single, double, triple, and quadruple excitations (MP4SDTQ) with the 6-31 1G**,4$b 6-31 1(+)G**,45cand 6-31 1G**(2dQ4" basis sets. Tocompensate for the residual correlation energy distributions, a further postMP4 calculation (quadratic configuration interaction including single, double, and triple excitations; QCISD(T)47) has been performed. The total energy calculated in the framework of the G1 theory is a sum of corrections to the MP4SDTQ/6-311GS* calculated energy. In the framework of the G2 theory, an additional MP2/6-3 11G** (3df,2p) calculation is required to make corrections to the G1 calculated energy. To overcome the remaining deficiencies in the basis set (higher polarization functions on non-hydrogen atoms) and to avoid the very extensive QCISD(T)/6-3 1 1(+)G**(2df) calculations, theso-called higher level correction term AE(HLC), introduced in a parametric way according to Pople's formula,44is used. The geometries given in the figures and discussed in the text refer to resultsofthe MP2(FULL)/6-31G** optimizationwithout any restrictions (Cl symmetry) as described earlier. If a higher symmetry resulted from the calculations, we have repeated the calculation in the corresponding point group. Bond lengths are given in angstroms and bond angles in degrees. In Table I both the G1 and G2 total energies are listed; the relative stabilities (in kcal/mol) discussed in the text refer only to G2 energies. All calculations have been performed on IBM/RISC 6000 work stations using the GAUSSIAN 92 program package.48 Theoretical Results The calculated post-MP2 total energies, a t various levels of theory, as well as the resulting G1 and G2 values are given in Table I. In agreement with previous a b initio studies,37-41 the isomers having the SiNHz connectivity represent the most stable species on both the ionic and the neutral potential energy surfaces. The MP2/6-3 1** calculated geometries of this isomer, which are in relatively good agreement with previous HF/6-3 1G calculations,39 are shown in Figure 1. For the neutral SiNH2* a Si-N bond length of 1.720 A is predicted. Upon ionization this distance is shortened by 0.06 A whereas the valence bond angle and the N-H bond remain practically unchanged. The stronger Si-N bond is a consequence of a stabilizing influence of the electron donation from the lone electron pair located a t the nitrogen atom into the empty *-orbital of silicon, on which the positive charge is located. It has previously been stated37.49 that, with respect to the silicon cation, the amino group acts as an extremely strong *-donor thus generating an SiN multiple bond. In contrast to the previously reported41nonplanar geometry of the SINHI' neutral (based on SCF/6-3 lG* calculations, the planar structure corresponds to a transition structure), our MP2(FULL)/6-3 1G** geometry optimization predicts the neutral and cationic SiNHz systems to be planar (Cbsymmetry). This finding is in agreement with the statement of Melius and Ho,41 who stressed the importance for the inclusion of correlation energy into the geometry optimization on such systems. Based on our calculations, a value of 155.7 kcal/mol results for the adiabatic ionization energy of SiNHZ' (eq 3): SiNH,'

-

SiNH:

+ e-

(3) From our previous G1 and G2 calculations on SiNH,19 for which the post-MP2 energies are also included in Table I, a temperature corrected value of 198.8 kcal/mol can be derived for the protonation at the nitrogen atom at 298 K. Our calculated PA is slightly smaller than the number resulting from bracketing experiments by Bohme20 ( P k X p= 203 f 2 kcal/mol). The next isomer derives from the silicon atom protonation of SiNH. The geometry optimization results in a planar structure (C,) for the neutral as well as the cationic HSiNH species (Figure

The Journal of Physical Chemistry. Vol. 97, No. 41, 1993 10689

Generation and Characterization of SiNH2+ and SiNH2'

H

H

\H

H Electronic

Electronic

1.616 1 4 8 6 1 541 1.456

1650 1.480

Figure 2. MP2/6-31G** oDtimized geometries for neutral (.2 A.,~ and ) two cationic and 'A') HSiNH n h m a .

Figure 1. MP2/6-31G** optimized geometries for neutral ( 2 A ~ and ) two cationic ('A1 and 'A,) SiNH2 minima.

calculated to be 21.8 kcal/mol less stable than the global minimum SiNH2'. The 'A' state of HSiNH+ is calculated to be 50.3 kcal/ mol higher in energy than the related global minimum (SiNH2+) on the singlet surface. The 3A' electromer of HSiNH+ is destabilized by an additional 42.9 kcal/mol. All computational attempts to find the as yet unknown cisHNSiH isomer on the neutral and both ionic PESs have been unsuccessful. For HSiNH' the only minimum found on the doublet surface possesses a linear SiNH substructure and an HSiN bond angle of 157.6'; the calculated SiN and N H bond lengths of 1.565 and 0.998 A are quite close to those in isolated SiNH ( ' S )(1.571 and 1.001 A), whereas the SiH bond (1.51 1 A) is elongated. As far as energy is concerned, this minimum is only 11.9 kcal/mol below the dissociation limit to generate SiNH + H'. For the cations a similar structure could not be located on either the singlet and triplet surfaces. Finally, the neutral and cationic H2SiN isomers have been studied. The MP2/6-31G** geometry optimization leads to planar minima (CzJ as shown in Figure 3. The neutral ZAlstate

2). The neutral radical has a "classical" trans structure as indicated by both bond angles (HSiN, 117.7'; SiNH, 119.9'). In the 'A'cationic form these angles are widened to 157.4O and 157.3O, respectively. This widening, with regard to the neutral, does indicate a significant rehybridization of the s and p orbitals on the heavy atoms. However, the linear HSiNH+ structure, reported by Florez and Largo-Cabreriz0,3* is associated with one imaginary frequency (180.8i cm-I) and thus does not represent a minimum on the PES. The 3A'structureof HSiNH+ is described by a long SiN bond (1.650 A). In comparison with the neutral structure, only the SiNH bond angle is widened up to 158.0', whereas the HSiN bond angle remains almost unchanged (1 15.4'). As mentioned earlier for the SiNH2 isomer, ionization of HSiNH' to the IA' HSiNH+ cation is also associated with a significant shortening of the SiN bond by 0.075 A; obviously, the increase in the SiN bond strength is due to a larger *-donation. With regard to the relative stabilities, the neutral HSiNH' is

TABLE I: Post-MP2 Calculated Total Energies (in hartrees) of the Neutral as Well as the Cationic Isomers on the [HtSiN] Potential Energy Surface and Their Fragments system

MP2

-288.892 Si SiH' 22+ -289.494 SiH+ lZ+ -289.216 SiH+ 'Z+ -289.141 SiH2IAl -290.110 -290.088 SiH23A1 SiH2'+ 2Al -289.797 SiN*2Z+ -343.493 SiN+ lZ+ -343.087 SiN+ ' 2 -343.038 SiNH lZ+ -344.219 SiNH 'A' -344.074 SiNH'+ *Z+ -343.805 HSiN 1 2 -344.127 HSiN 'A' -344.036 HSiN'+ 2Z+ -343.174 SiNH2'2Al -344.785 SiNHz+ 'A1 -344.546 SiNH2+ )Al -344.429 HSiNH'2A' -344.745 HSiNH+ IA' -344.465 HSiNH+ 'A' -344.393 H2SiN' 2Al -344.681 H2SiN+ 'A1 -344.254 H2SiN+ 'Al -344.306 H2SiN+ 3B2 -344.319 H2SiN+'A2 -344.353 -344.695 TS1 'A' -344.421 T S l + 'A' -344.332 T S l + 'A' -344.621 TS2 'A' -344.312 TS2+ 'A' -344.281 TS2+ 3A'

MP2(+) 196 -288.892 834 -289.495 926 -289.217 820 -289.142 806 -209.111 715 -290.089 225 -289.797 901 -343.497 358 -343.088 909 -343.039 249 -344.224 479 -344.079 937 -343.807 165 -344.129 685 -344.042 025 -343.715 832 -344.791 872 -344.549 685 -344.431 055 -344.752 634 -344.467 964 -344.398 723 -344.686 306 -344.255 350 -344.308 319 -344.320 138 -344.354 120 -344.703 368 -344.423 760 -344.335 469 -344.628 834 -344.314 668 -344.283

MP2(2df)

494 -288.906 470 -289.510 305 -289.225 135 -289.149 472 -290.128 078 -290.103 508 -289.806 604 -343.541 766 -343.128 819 -343.080 235 -344.268 934 -344.120 629 -343.850 805 -344.173 766 -344.086 535 -343.758 192 -344.832 31 1 -344.588 882 -344.469 045 -344.796 288 -344.509 887 -344.435 356 -344.730 775 -344.297 015 -344.348 601 -344.360 832 -344.392 326 -344.745 533 -344.466 501 -344.372 042 -344.673 451 -344.356 611 -344.322

613 744 698 109 270 464 943 071 266 859 090 647 275 120 594 660 968 566 779 915 112 792 232 683 557 544 936 717 410 648 736 007 919

MP2/(3df,2p)

MP4

-288.907 247 -289.513 094 -289.227 503 -289.150 269 -290.131 939 -290.106 338 -289.809 294 -343.548 520 -343.133 453 -343.058 724 -344.276 474 -344.127 999 -343.857 234 -344.178 347 -344.094 099 -343.764 379 -344.842 355 -344.596 470 -344.477 016 -344.806 791 -344.516 706 -344.442 521 -344.738 148 -344.303 345 -344.354 353 -344.366 210 -344.398 608 -344.755 839 -344.473 573 -344.378 893 -344,683 835 -344.362 313 -344.328 717

-288.907 -289.515 -289.237 -289.156 -290.136 -290.104 -289.815 -343.527 -343.135 -343.066 -344.247 -344.106 -343.829 -344.161 -344.070 -343.749 -344.815 -344.576 -344.452 -344.777 -344.495 -344.424 -344.721 -344.295 -344.348 -344.355 -344.389 -377.729 -344.459 -344.370 -344.662 -344.356 -344.321

MP4(+)

MP4(2df)

784 -288.908 066 -288.925 549 -289.516 185 -289.534 683 -289.238 074 -289.247 130 -289.156 459 -289.165 137 -290.136 840 -290.157 306 -290.108 908 -290.126 844 -289.816 167 -289.827 176 -343.531 101 -343.577 037 -343.136 391 -343.176 162 -343.067 204 -343.109 368 -344.251 917 -344.298 877 -344.1 12 563 -344.157 575 -343.831 255 -343.876 765 -344.164 321 -344.211 108 -344.076 063 -344.123 824 -343.751 423 -343.797 760 -344.821 217 -344.867 821 -344.579 336 -344.620 429 -344.454 641 -344.495 087 -344.783 609 -344.831 022 -344.496 506 -344.540 753 -344.426 779 -344.469 21 1 -344.726 122 -344.774 674 -344.291 342 -344.343 935 -344.350 793 -344.394 108 -344.357 114 -344.400 272 -344.391 182 -344.432 401 -344.737 485 -344.783 996 -344.462 096 -344.507 390 -344.373 323 -344.413 863 -344.669 471 -344.718 873 -344.358 653 -344.403 365 -344.323 550 -344.366

G1

G2

447 -288.909 317 -288.939 949 -289.518 212 -289.546 386 -289.240 568 -289.258 066 -289.158 805 -289.170 010 -290.139 134 -290.168 656 -290.1 10 495 -290.138 509 -289.818 014 -289.830 633 -343.543 052 -343.619 398 -343.156 670 -343.221 612 -343.078 665 -343.132 954 -344.244 945 -344.320 734 -344.113 669 -344.184 192 -343.839 725 -343.901 580 -344.141 342 -344.216 514 -344.078 443 -344.154 033 -343.767 401 -343.833 314 -344.817 680 -344.882 619 -344.578 622 -344.633 377 -344.454 934 -344.502 960 -344.779 992 -344.849 672 -344.492 839 -344.552 931 -344.429 570 -344.484 385 -344.737 854 -344.810 050 -344.314 225 -344.377 731 -344.358 291 -344.421 363 -344.363 207 -344.417 934 -344.393 090 -344.447 820 -344.733 023 -344.810 212 -344.464 437 -344.527 323 -344.376 524 -344.433 966 -344.680 643 -344.759 339 -344.355 774 -344.421 010 -344.333 380 -344.392

QCISD(T)

541 -288.937 358 -289.545 272 -289.257 514 -289.169 082 -290.167 652 -290.138 871 -289.830 171 -346.618 532 -343.220 397 -343.134 465 -344.318 852 -344.182 361 -343.902 888 -344.213 027 -344.150 643 -343.833 738 -344.881 185 -344.632 054 -344.502 130 -344.846 506 -344,552 194 -344.484 488 -344.808 491 -344.375 242 -344.419 502 -344.417 564 -344.446 986 -344.807 069 -344.526 085 -344.432 753 -344.757 047 -344.420 195 -344.391

597 792 419 079 665 884 659 356 751 072 163 189 069 775 891 293 064 949 533 316 746 440 071 983 674 386 981 204 367 029 580 035 491

Goldberg et al.

10690 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993

H k - N

/

H

Electronic State

RSiH

a

1.615

1.468

123.2

1.705

1.459

113.8

1.623

1.465

114.1

SiN -

1.818

1.458

120.5

1.780

1.457

118.9

1.650

1.454

114.0

1.676

1.457

118.9

- -Figure3. MP2/6-31G** optimizedgeometriesfortheneutraland several

cationic H2SiN minima. is characterized by a Si-N distance of 1.615 8,(bond angle HSiN, 123.3'). Both bond parameters change dramatically if this radical ~ ~ the cationic system is ionized. As stated p r e v i o ~ s l y ,for H2SiN+ different electronic states of comparable energies have to be taken into account. At our level of theory the most stable electromer corresponds to the 3A2state (...7a122b113b21)with a SiN bond length of 1.780 A and an HSiN bond angle of 118.9'. Thesecondlow-lying tripletstate3B2(...2b127a113b21) isdescribed by a SiN bond length of 1.650 A and a bond angle of 114.0'. The lowest singlet state 1Al (...2b227aI22bl2)possesses a SiN bond lengthof 1.705Aandabondangleof 113.8'. The'Al electromer (...2b227a123b22)can be considered as a predissociative state with regard to its long SiN distance (1.818 A) and a bond angle of 120.5'. Finally, two other electromers, i.e. 'B1 (...3b227a112b11) and 'AI (...2b222b123b2~),havebeenlocatedat theMP2/6-31G** potential energy surface. The geometries are given in Figure 3. As reported previously,3840 the 3Bl and 'AI states are very high inenergy(92.1 kcal/mol (3Bl),171.9 kcal/mol (IAl)) withrespect to the most stable 3A2state. For this reason and in view of the instability of the wave function, we did not attempt to calculate the G2 energies of these electromers.50 As summarized in Figure 3, the HSi bond length is not very sensitive to changes in the electronic structure and remains almost the same in all systems studied. On the cationic PES the most stable electromer (3A2) of H2SiN+ is found to be 116.7 kcal/mol less stable than the global minimum (SiNH2+). The lowest singlet state ('AI) lies 16.3 kcal/mol, the next triplet state (3B2) 18.6 kcal/mol, and finally the second IAl state 44.6 kcal/mol above the 3A2state. These calculated excitation energies are in rough agreement with the relative stabilities calculated earlier38at MP4/6-3 1G(dp)//HF/ 6-31G. In view of the assumed existence of the neutral and/or ionic [H2SiN] molecules in the interstellar or circumstellar space, a knowledge of some basic properties of these systems is required. In Table I1 the calculated, unscaled harmonic frequencies for all minima found on the neutral and both cationic surfaces are given. Because of the lack of experimental data for [H2SiN], comparison can only be made with the matrix spectra of SiNH = 1.54

A, R N H=

1.005 A1.13

The calculated N H stretching frequencies for both SiNH2* and HSiNH' compare well with themeasuredvalueof 3583 cm-1 for SiNH. The experimental value for the SiN stretching mode (1 198 cm-I) is similar to the calculated frequency in HSiNH, reflecting the small differences in the SiN bond length in the 2A'

(Ar = 0.06 A) and 'A' (Ar = 0.001 A) with respect to neutral SiNH. The SiN stretching vibration in the 'A' state of HSiNH' is shifted to 1057.4 cm-I, consistent with the elongation of the SiN bond length. In general, the differences in the SiN stretching vibrations of all systems under study indicate a large variation in the bond order of the silicon-nitrogen bond. The calculated SiH2 stretching modes in neutral and ionic H2SiN' are not very sensitive to the electronic state, reflecting only small changes in the SiH bond length. All values are close to 2400 cm-1, and after scaling by a factor 0.893, the comparison with experimental values (for example, 2224 cm-I in H2SiI2 and 2228 cm-l in H3SiCCH)51 is not too bad. With respect to the possible intramolecular rearrangements of the described isomers, we have also calculated the barriers for the hydrogen migration on the neutral and on both cationic surfaces. The optimized geometries of the transition structures TS1 (eq 4) and TS2 (eq 5) are shown in Figures 4 and 5.

SiNH,'

- - TS 1

t-HSiNH'

TS2

t-HSiNH'

(4)

H,SiN'

(5)

In both the neutral and cationic systems, TS1 is a classical hydrogen-bridged structure, as can be deduced from the elongation of the corresponding SiN and SiH bonds with respect to the related trans minimum of HSiNH'. In all cases the imaginary component in the frequency analysis lies within the expected range for hydrogen migrations known from our earlier studies43of similar systems (2438.4icm-I (2A'), 2014.2 cm-l (lA'), and 2254.8 cm-I (3A')). For the neutral case the barrier (UTSI) is calculated to be 45.0 kcal/mol. Analogous structures can be located for the singlet and triplet cationic PES (Unl = 66.9 kcal/mol (1A') and UTSI = 126.1 kcal/mol (3A')). Similar conclusions can be made for TS2 connecting the t-HSiNH' and the H2SiN' minima. Both the neutral as well as the cationic cases are predicted to be planar (C,) and possess SiNH bond anglesof approximately 55' (56.8' (2A'), (IA') 55.6O (3A')) and 50.2O (Figure 5). Again, the SiN bond in the 3A' state is significantly elongated to 1.735 A. As expected, the calculated imaginary frequencies of TS2 associated with the hydrogenmigration aresmaller (1736.licm-I (2A'), 1810.6icm-1 (3A'), and 1098.83 cm-I (IA')) than for the corresponding TS1. It should be mentioned that the frequency for the IA' state is noticeably smaller than expected. However, the SiNH'+ H' exit channel lies only 11.0 kcal/mol higher in energy, thus indicating a quite flat PES in this region, which complicates the search for the transition structures.

+

Mass Spectroscopic Results The experimentally generated [H2SiN]+ions most likely result from multistep reactions. The differences in the ionization energies (IE(NH3) = 10.16 eV, IE (SiHJ) = 9.5 eV)s2suggest that the reaction sequence may commence with the radical cation SiH3I*+for which several pathways are conceivable to eventually give rise to [H2SiN]+. Mechanistic details on the genesis of the ions are not available yet. For a structural characterization, the beam of mass-selected [H2SiN]+ ions was subjected to a CA experiment. The spectrum is given in Figure 6. The base peak (m/z 43) in the CA spectrum corresponds to the H' loss resulting in the formation of the afore-mentioned SiNH*+ion.19 A further hydrogen loss gives rise to SiN+ (m/z 42). Alternatively, the Occurrence of this ion can be due to the H2 elimination. The lower mass region, displayed in the spectrum, is dominated by the signal for the Si*+ion (m/z 28). This shows that the NH2' elimination, a structure-indicative process for the ion having the Si-NH2+ connectivity, represents a major exit channel for the system studied. Although weak, the appearance of a signal at m/z 16 (NH2+) also is in support of our assignment.

Generation and Characterization of SiNH2+ and SiNH2'

The Journal of Physical Chemistry, Vol. 97, No. 41, 1993

H

10691

SiNH"

/

H'

Figure 6. Collisional activation mass spectrum of SiNH2+ (collision gas, helium; 80%T). The signal intensities at m/z 16 and m / z 22 are blown up by a factor of 10. Figure 4. MP2/6-31G** optimized geometries for the neutral and the twocationic transition structures ( ' E l ) connectingtheSiNH2and HSiNH minima.

Recovery Signal SiNH,'

H' NH"

/

NH:

\ /

11111

x5

SiNH"

H

~ i R_ ~,N R~ SH~ TS2 *A' 1.566

,R

p

a

1.493

1.613

144.5 56.8

TS2+ 3A' 1.735

1.474

1.574

157.3 55.6

TS2+ 'A, 1.654

1.459

1.868

117.7 50.2

Figure 7. Neutralization-reionization mass spectrum of SiNHZ+(collision gases, xenon, 80%T; oxygen, 80%T). The signal intensities in the region m / z 14-16 are blown up by a factor of 5.

the third field-free region of the four-sector spectrometer. The ions were first neutralized with xenon and then reionized with oxygen according to eq 6.

Figure 5. MP2/6-31GS* optimized geometries for the neutral and the two cationic transition structures (TS2) connecting the HSiNH and HzSiN minima.

- Xe

The weak signals at m / z 29 and 30 (SiH+ and SiH2*+) are in keeping with a collisional isomerization SiNH2+ -* TS1 -* HSiNH+-* H*SiN+preceding dissociation. In addition, a signal for the doubly charged SiNH2'*+ ion ( m / z 22) resulting from a charge stripping process is observed.53 Fuqther support for our suggestion that we are dealing with SiNH2+is provided by a neutralization-reionization experiment; here, the ion beam was subjected to two consecutive collisions in

SiNH:

0 2

SiNH,'

SiNH:

Reduction of SiNH2+ in the N R M S experiment (Figure 7) gives rise toanintenserecoverysignalatm / z 4 4 . Thisobservation, quite likely, reflects both the stability of the neutral radical and points to relatively favorable Franck-Condon factors in thevertical electron-transfer process.54 Further, strong signals appear at m / z 43 and 42 representing the H' and Hz losses. Structure-indicative

TABLE 11: MP2/6-31C** Calculated Harmonic Frequencies (Unscaled) in cm-l for the Neutral and the Cationic Isomers on the [HfiiN] Potential Energy Surface

SiNH2' (*AI) SiNH2+ (IAl) SiNH2+ ('Al)

NH2 stretch b2

NH2 stretch a l

NH2 bending al

SiN stretch a l

NH2 out-of-plane bl

NH2 wagging b2

3736.3 3636.3 3701.2

3623.3 3535.3 3575.4

1630.3 1580.8 1595.0

875.6 98 1.4 996.9

763.0 700.8 898.9

601.4 596.4 520.9

~

HSiNH' (2A') HSiNH+ ('A') HSiNH+ ('A')

H2SiN' ( 2 A ~ ) HZSiN+ ('AI) H2SiNt ('AI) H2SiN+ (IAl) H2SiN+ ()A2) H2SiNt OB2) H2SiN+ P B I )

NH stretch a'

SiH stretch a'

SiN stretch a'

sYm bend a"

sYm bend a'

asym bend a'

3637.6 3712.4 3673.3

2301.5 2422.9 2258.4

1258.9 1266.6 1057.4

1135.4 639.5 755.0

935.1 620.1 552.1

795.6 269.4 315.4

SiH2 stretch b2

SiH2 stretch a l

SiH2 bending al

SiN stretch a l

2395.5 2461.4 2479.4 2407.7 2486.6 2494.1 2481.2

2368.1 2389.9 2420.9 2319.6 2422.7 2417.4 2414.4

994.1 989.9 1048.4 998.4 984.3 1085.7 1038.6

914.3 785.7 774.1 913.4 784.7 896.9 990.4

SiH2 out-of-plane bl 806.3 628.6 714.8 751.1 655.6 699.9 775.9

SiH2 wagging b2 554.7 317.3 663.1 717.1 550.5 618.4 766.2

Goldberg et al.

10692 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993

Io:!

SiH, + N +

kcall

S"

!

+ NH+'

kcal I

=I

SiH'+ HN

r q/p

SiH2+N'

HSN + H+

si+ NH;

zq

HSiN +H'

Si + NH;

7

SiHi'+ N' HSiN+'+ H'

Si"+

H,

7 1

SiNH + H '

I SiNH; 0.0

Figure 9. The G2 calculated potential energy surface for the neutral [H2SiN]' system (see also the figure caption of Figure 8). Figure 8. The G2 calculated potential energy surface for the cationic [H2SiN]+ system. Possible transition structures associated with the dissociation processes have not been calculated.

fragmentations are the strong Si'+ signal ( m / z 28) and the fingerprint signal of NH2+. Interestingly, no ions whatsoever a t m / z 29 and 30 (SiH+ and SiH2'+) are observed. This clearly points to the fact that the observed neutral species does not contain silicon-hydrogen bonds, thus ruling out the connectivities HSiNH' and/or H2SiN'.

Discussion The ab initio calculations predict SiNH2+ as the most stable isomer on the [HzSiN]+ potential energy surface, and this result is in keeping with the mass spectrometric study. Further insight into the intrinsic properties of the [H2SiN]+ system can be obtained by combining the calculated thermochemical stabilities and the results of the CA experiments. The fragmenation pattern of the collisional activation spectra is in good agreement with the theoretical findings. The two dominant reactions correspond to the H* loss and the NH2' elimination. These two reactions are associated with the two lowest-lying dissociation channels on the calculated cationic potential energy surface (Figure 8). The fact that the H' loss which was calculated to lie 13.2 kcal/mol higher in energy than the competing NH2' loss gives rise to a more intense signal (Figure 6) can be explained by the existenceof a barrier connected with the electronic rearrangement involved in the dissection of the silicon-nitrogen multiple bond of SiNHZ+. We have not tried to calculate this stationary point as this would exceed the possibilities of the single-determinant based a b initio method employed in the present study. All other fragments and their relative intensities in the CA spectra can be explained in a straightforward manner. The weak signals at m / z 29 and m / z 30, corresponding to SiH+ and SiH2'+, quite likely result from a collision-induced isomerization precedingdissociation. There exist many precedents for this behavior, and the low-lying exit channels for SiH+ + NH (146.8 kcal/mol) and SiHZ'+ + N' (196.6 kcal/mol) do not rule out this interpretation. Nevertheless, if the experimentally generated [H2SiN]+ species would preferentially correspond to HSiNH+ or H2SiN+, the SiH2*+and SiH+ ions should play a far more dominant role in the collisional activation processes. This

is not observed. The NH2+ signal at m / z 16 corresponds to the loss of atomic silicon, which has been calculated to lie 225.8 kcal/mol higher in energy than the global minimum. In view of the huge energy demand associated with the formations of SiH' NH*+and SiH2' N + (275.3 and 300.4 kcal/mol, respectively), it is of no surprise that the corresponding signals at m / z 15 and m / z 14 are not present in the CA spectrum. Previous studies have clearly indicated that the neutralizationreionization technique provides convincing information about the existence of bound neutral species, on the condition that a strong recovery signal is present in the N R spectra. Furthermore, in cases where the transition structures for the intramolecular rearrangements are not significantly lower than the possible exit channels, the N R process leads to the formation of well-defined, structure-indicative product i0ns.~3a According to the ab initio MO calculation, the by far lowest dissociation process in the [H2SiN]' system corresponds*to H' loss, for which a minimal energy of 39.5 kcal/mol is predicted. This fact may explain the strong signal of SiNH+ observed in the N R spectrum. The Si'+ signal corresponding to NHz' loss has been calculated to require 98.6 kcal/mol starting from SiNH2'; this channel represents the second-lowest path for the fragmentation of the SiNHz molecule. The fact that the N R spectrum displays the NH,+ ions ( x = 0-2) with higher relative intensities than in the corresponding CA spectrum is also in keeping with the details of the two PESs. On the neutral surface thesechannels are far moreeasier to be reached by the system thanin thecationic case. In addition, the absence of signals for SiH,+ ( x = 1, 2) in Figure 9 follows directly from the nature of the PES. It should be recalled that the CA and N R mass spectra of SiNH2+display,in their principal features, a pattern very similar to that of the previously investigated [HSiN]+ system.lg For this system all experimental and theoretical evidence points to the connectivity SiNH*/+rather than HSiN*/+. The fact that in the [HzSiN] system all other precursors studied give rise to CA and NR spectra identical with the ones shown in Figures 6 and 7 strongly suggests that the initially formed [H2SiN]+ions most likely isomerize to its global minimum structure SiNH*+ which is probed experimentally. Thus, we conclude that the present work presents at least circumstantial evidence for the existence of the long-sought-after SiNH2+ and SiNH2+ molecules.

+

+

Generation and Characterization of SiNH2+ and SiNH2'

Acknowledgment. We acknowledge the generous financial support of our work by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. H.S. is grateful to the Alexander von Humboldt-Foundation for a Max Planck Research Award (jointly with Professor C. Lifshitz) which provides the financial basis for his collaboration with the Hebrew University of Jerusalem. References and Notes (1) Bohme, D. K. In Structure/Reactivity and ThermochemistryofIons; Ausloos, P., Lias, S.G., Eds.; Reidel: Dordrecht, 1987; p 219. (2) Bohme, D. K.; Wlodek, S.;Fox, A. In Rate Coefficients in Astrochemistry; Millar, T. J., Williams, D. A,, Eds.; Kluwer: Amsterdam, 1988; p 193. (3) Wlodek, S.;Bohme, D. K. J . Am. Chem. SOC.1988, 110, 2396. (4) Bohme, D. K.; W1odek.S.; Wince], H. Astrophys. J . 1989,342,191. (5) Herbst, E.;Millar,T. J.; Wlodek,S.;Bohme, D. K. Astron. Astrophys. 1989, 222, 204. (6) Bohme, D. K. Int. J. Mass Spectrom. Ion Processes 1990,100,719. (7) Glassgold, A. E.; Manson. G. A. In Chemistry and Spectroscopy of Interstellar Molecules; Bohme, D. K., Herbst, E., Kaifu, N., Saito, S., Eds.; University Tokyo Press: Tokyo, 1992; p 261. ( 8 ) Turner, J. L.; Dalgarno, A. Astrophys. J . 1977, 213, 386. (9) Clegg, R. E. S.;van Ijzendoorn, L. J.; Thaddeus, P.; Allamandola, L. J. Mon. Nor. R. Astr. SOC.1983, 203, 125. (10) Cernicharo, J.; Gottlieb, C. A.; Guelin, M.; Thaddeus, P.; Vrtilek, J. M. Astrophys. J. 1989, 341, L25. (1 1) Lim, K. P.; Lampe, F. W. Inr. J . MassSpectrom. Ion Processes 1991, 101, 245. (12) (a) Luke, B. T.; Pople, J. A.; Krogh-Jespersen, M.-B.; Apeloig, Y.; Karni, M.; Chandrasekhar, J.;Schleyer, P. v. R. J.Am. ChemSoc. 1986,108, 260. (b) Luke, B. T.; Pople, J. A,; Krogh-Jespersen, M.-B.; Apeloig, Y.; Karni, M.;Chandrasekhar, J.;Schleyer, P. v. R. J . Am. Chem. SOC.1986,108, 270. (c) Patai, S.;Rappoport, Z. The Chemistry of Functional Groups: The Chemistry ofSilicon Compounds;Wiley Interscience: New York, 1989;Parts 1 and 2. (13) Ogilvie, J. F.; Cradwk, S. Chem. Commun. 1966, 12, 364. (14) Maier, G.; Glatthaar, J.; Reisenauer, H. P. Chem. Ber. 1989, 122, 2403. (15) Lovas, F. J. Asrrophy. J . 1974, 193, 265. (16) Elhamine, M.; Farrenq, R.; Guelachvili, G. J. Chem. Phys. 1991,94, 2529. (17) Bogey, M.; Demuynck, C.; Destombes, J. L.; Walters, A. Astron. Astrophys. 1991, 244, L47. (18) (a) Kroto, H. W.; Murell, J. N.; AI-Derzi, A.; Guest, M. Astrophys. J . 1978, 219, 886. (b) Murrell, J. N.; Kroto, H. W. J. Chem. SOC.Chem. Commun. 1977, 619. (19) Goldberg, N.; Iraqi, M.; HruSdk, J.; Schwarz, H. Int. J. Mass Spectrom. Ion Processes 1993, 125, 267. (20) Bohme, D. K. Adv. Gas Phase Ion Chem. 1992, 1, 225. (21) Terlouw,J.K.;Burgers,P.C.;vanBaar, B.L.M.; Weiske,T.;Schwarz, H. Chimia 1986, 40, 357. (22) Wesdemiotis, C.; McLafferty, F. W. Chem. Rev. 1987, 87, 485. (23) Terlouw, J. K.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1987,26, 805. (24) Schwarz, H. Pure Appl. Chem. 1989, 61, 685. (25) Holmes, J. L. Mass Spectrom. Rev. 1989, 8, 513. (26) Terlouw, J. K. Ado. Mass Spectrom. 1989, 11, 984. (27) McLafferty, F. W. Science 1990, 247, 925. (28) McLafferty, F. W. Int. J. Mass Spectrom. Ion Processes 1992.1181 119, 221. (29) Srinivas, R.; Siilzle, D.; Schwarz, H. Chem. Phys. Lett. 1990, 175, 575.

The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10693 (30) Srinivas, R.;Siilzle, D.; Koch, W.; DePuy,C. H.;Schwarz, H. J . Am. Chem. SOC.1991, 113, 5970. (31) Srinivas, R.; Bbhme, D. K.; Siilzle, D.; Schwarz, H. J . Phys. Chem. 1991, 95, 9836. (32) Srinivas, R.;SCllzle, D.; Weiske,T.;Schwarz, H. Int. J . MassSpectrom. Ion Processes 1991, 107, 369. (33) Srinivas, R.; Bbhme, D. K.; Hruidk, J.; Schrider, D.; Schwarz, H. J. Am. Chem. SOC.1992, 114, 1939. (34) Srinivas, R.; HruSdk, J.; SBlzle, D.; Bbhme, D. K.; Schwarz, H. J. Am. Chem. SOC.1992, 114,2920. (35) Iraqi, M.; Schwarz, H. Chem. Phys. Lett. 1993, 205, 183. (36) See, for example, the excellent, exhaustive review of Y. Apeloig in ref 12c, p 57. (37) Hopkinson, A. C.; Lien, M. H. Can. J. Chem. 1989, 67, 991. (38) Flores, J. R.; Largo-Cabrerizo, J. J. Mol. Struct. (THEOCHEM) 1989, 183, 17. (39) Flores, J. R.; Largo-Cabrerizo, J. Chem. Phys. Lett. 1987,142, 159. (40) Flores, J. R.; Gomez Crespo, F.; Largo-Cabrerizo, J. Chem. Phys. Lett. 1988, 147, 84. (41) Melius, C. F.; Ho, P. J . Phys. Chem. 1991, 95, 1410. (42) (a) Radom, L. Org. Mass Spectrom. 1991,26,359. (b) Radom, L. Int. J. Mass Spectrom. Ion Process 1992, 1181119, 339. (43) (a) Grandinetti, F.; HruSgk, J.; Schrider, D.; Schwarz, H. J. Phys. Chem. 1992,96,2102. (b) Grandinetti, F.; HruSdk, J.; Schrider, D.; Karass, S.;Schwarz, H. J . Am. Chem. SOC.1992, 114, 2806. (c) SchrBder, D.; Grandinetti, F.; HruSdk, J.; Schwarz, H. J . Phys. Chem. 1992, 96, 4841. (44) (a) Pople, J. A.; Head-Gordon, M.; Fox, D. J.; Raghavachari, K.; Curtiss, L. A. J . Chem. Phys. 1989, 90, 5622. (b) Curtiss, L. A.; Jones, C.; Trucks, G. W.; Raghavachari, K.; Pople, J. A. J. Chem. Phys. 1990,93,2537. (c) Curtiss, L. A.; Carpenter, J. E.; Raghavachari, K.; Pople, J. A. J. Chem. Phys. 1990,93,2537. (d) Curtiss, L. A.; Raghavachari, K.; Trucks, G. W.; Pople, J. A. J. Chem. Phys. 1991, 94, 7221. (45! (a) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973,28, 213. (b) Krishnan, R.; Binkley, J. S.;Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (c) Frisch, M. J.; Pople, J. A.; Binkley, J. S.J. Chem. Phys. 1984, 80, 3265. (46) M~ller.C.: Plesset. M. S.Phvs. Rev. 1934. 46. 618. (47j Pople, J. A.; HeadLGordon, M,;Raghavachari,' K. J. Chem. Phys. -i9n7 _ _ .87 -. , , 5968 (48) GAUSSIAN 92, Revision B: Frisch, M. J.; Trucks, G. W.; HeadGordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.;Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.;Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.;Stewart, J. J. P.;Pople, J. A. GaussianInc.,Pittsburgh, PA, 1992. (49) Preuss, R.; Buenker, R. J.; Peyerimhoff, S. D. J. Mol. Strucr. (THEOCHEM)1978.49, 171. (50) The single-determinant based calculations on excited states of H2SiN+ resulted in a wave functions which have an internal instability with significant participations of all excitedconfiguration. Furthermore, the higher excited states ('A, and SB1) have larger MO coefficients (typically by a factor of 1.2). (51) Goldberg, N. Diploma Thesis, Universitlt Giessen, 1991. (52) Lias, S.G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17 (Suppl. 1). (53) It should be mentioned that all attempts to generate one or the other isomer of SiNHZ+failed. Even the [H2SiN]+ion generated by electron impact ionization of (H,Si)3N displayed CA and NR mass spectra practically identical with the spectra shown in Figures 6 and 7. The formation and characterization of gaseous HSiNH- anions was recently described (Damrauer, R.; Krempp, M.; OHair, R. A. J. J. Am. Chem. SOC.1993, 115, 1998). Experiments aimed at using this anion as a precursor to generate HSiN' failed on sensitivity grounds. (54) (a) Fournier, P.; Appell, J.; Fehsenfeld, F. C.; Durup, J. J . Phys. B 1972,5, L58. (b) Lorquet, J. C.; Ley-Nihaut, B.; McLafferty, F. W. Inr. J. Mass Spectrom. Ion Processes 1990, 100, 465.