Reactions of laser-ablated scandium atoms with nitrogen: Matrix infrared spectra and DFT calculations for scandium nitrides and the fixation of nitrogen by two scandium atoms

Article abstract of DOI:10.1021/ja973701d

Laser-ablated Sc atoms react with nitrogen to form ScN, Sc(N₂) and (ScN)₂ and their N₂-ligated counterparts. These identifications are based on nitrogen-15 isotopic shifts and agreement with DFT vibrational frequency calculations. It is noteworthy that annealed argon matrix samples containing 2% N₂ give almost the same absorptions as nitrogen matrix samples for the ligated complexes (NN)(x)ScN and (ScN)₂(NN)(x). The rhombic (ScN)₂ molecule is made by reaction of a second Sc atom with an intermediate ScN₂ species and, thus, provides a method of nitrogen fixation with naked metal atoms.

Full text of DOI:10.1021/ja973701d

J. Am. Chem. Soc. 1998, 120, 3205-3212
3205
Reactions of Laser-Ablated Scandium Atoms with Nitrogen: Matrix
Infrared Spectra and DFT Calculations for Scandium Nitrides and the
Fixation of Nitrogen by Two Scandium Atoms
George V. Chertihin,† Lester Andrews,*,† and Charles W. Bauschlicher, Jr.‡
Contribution from the Department of Chemistry UniVersity of Virginia, CharlottesVille, Virginia 22901,
and STC-230-3, NASA Ames Research Center, Moffett Field, California 94035
ReceiVed October 24, 1997
Abstract: Laser-ablated Sc atoms react with nitrogen to form ScN, Sc(N2) and (ScN)2 and their N2-ligated
counterparts. These identifications are based on nitrogen-15 isotopic shifts and agreement with DFT vibrational
frequency calculations. It is noteworthy that annealed argon matrix samples containing 2% N2 give almost
the same absorptions as nitrogen matrix samples for the ligated complexes (NN)xScN and (ScN)2(NN)x. The
rhombic (ScN)2 molecule is made by reaction of a second Sc atom with an intermediate ScN2 species and,
thus, provides a method of nitrogen fixation with naked metal atoms.
Introduction
observed at 938.0 cm-1 in solid argon and at 934.8 cm-1 in
solid nitrogen based on nitrogen isotopic shifts.13 Subsequent
work with Ti, V, Cr, Mn, and Co, Ni has identified both the
isolated MN molecules as well as a series of (NN)xMN
complexes in solid argon and the saturated dinitrogen complexes
in solid nitrogen from their M-N stretching frequencies and
nitrogen isotopic shifts.14-16 A major product in the Sc
experiments is the rhombic ring (ScN)2, which is formed by
reaction of two Sc atoms with one dinitrogen molecule and is
relevant to the nitrogen fixation problem.
The important role of transition metal centers in the nitrogen
fixation process has been the goal of much significant research.
A lot of this work has involved coordination chemistry of Mo,
V, and Fe as related to the function of the active center of
nitrogenase.17 End-on and side-on bonding of dinitrogen to
early transition-metal dinuclear complexes has been explored,
particularly with zirconium.18,19 Dinitrogen cleavage by Mo-
(III) complexes has recently been reported.20 It is clear from
these and many other works that two metal centers may be
required for efficient cleavage of the dinitrogen molecule. We
report here simple reactions of naked scandium atoms with N2
and show that two Sc atoms can react with N2 to form the
rhombic molecule (ScN)2 where the dinitrogen bond is broken.
Little is known experimentally about first-row transition metal
nitride molecules with the exception of TiN,1,2 VN,3,4 and CrN,5
and only mass and emission spectra with rotational analysis6,7
have been reported for molecular ScN. The solid material is
hard, refractory, and electrically conducting.8,9 The most recent
ab initio calculations predict a formal triple bond10 and a
harmonic frequency near 770 cm-1 for ScN although earlier
calculations revealed only weak bonding.11 Scandium holds a
special place in the role of transition metal chemistry because
Sc contains the smallest number of electrons, there are three
low-lying electron configurations (3d14s2, 3d24s1, 3d14s14p1),
and the 4s and 3d orbitals have similar radial extent and may
simultaneously participate in bonding. This latter point is
demonstrated by the recent observation of scandium dioxide,
OScO, with an antisymmetric stretching fundamental between
that for OTiO, where double bonds are present, and for OCaO,
where only single bonds can form.12
Laser-ablated transition metal atoms contain sufficient excess
energy to dissociate molecular nitrogen, and direct metal and
nitrogen atom combination during sample condensation leads
to the formation of MN molecules in both argon and nitrogen
matrixes. In the first application of this technology, FeN was
Experimental Section
† University of Virginia.
The technique for laser ablation of transition metal atoms and FTIR
matrix investigation of reaction products has been described in detail
‡ NASA Ames Research Center.
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100, 14609.
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44.
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13/14, 365
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Harrison, J. F. J. Phys. Chem. 1996, 100, 3513.
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(12) Chertihin,. G. V.; Andrews, L.; Rosi, M.; Bauschlicher, C. W., Jr.
J. Phys. Chem. A. 1997, 101, 9085.
(14) Andrews, L.; Bare, W. D.; Chertihin, G. C. J. Phys. Chem. A 1997,
101, 8417.
(15) Kushto, G. P.; Souter, P. F.; Chertihin, G. C.; Andrews, L. To be
published.
(16) Andrews, L.; Citra, A.; Chertihin, G. V.; Bare, W. D.; Neurock,
M. J. Phys. Chem. A. To be published.
(17) Richards, R. L. Coord. Chem. ReV. 1996, 154, 83 and references
therein.
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Am. Chem. Soc. 1994, 116, 9329.
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H.; Rettig, S. J. Am. Chem. Soc. 1993, 115, 2782.
(20) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim,
E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem. Soc.
1996, 118, 8623 and references therein.
S0002-7863(97)03701-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/18/1998

3206 J. Am. Chem. Soc., Vol. 120, No. 13, 1998
Chertihin et al.
Figure 1. Infrared spectra in the 2350-1550 cm-1 region for laser-
ablated scandium atoms deposited with 2% N2 in argon: (a) after
deposition for 1 h at 10 K; (b) after annealing to 25 K; (c) after
annealing to 30 K; (d) after annealing to 40 K.
previously.12-14,21-23 The scandium target (Johnson-Matthey, lump) was
mounted on a rotating (1 rpm) stainless steel rod. The Nd:YAG laser
fundamental (1064 nm, 10 Hz repetition rate, 10 ns pulse width) was
focused on the target through a hole in a CsI cryogenic (10 K) window.
Laser power ranged from 40 to 50 mJ/pulse at the target. Metal atoms
were co-deposited with 1-2% N2/Ar mixtures or with pure N2 (and
isotopic modifications and mixtures) for 1-2 h periods. FTIR spectra
were recorded with 0.5 cm-1 resolution and (0.1 cm-1 frequency
accuracy on a Nicolet 750 spectrometer using a 77K MCTB detector.
Matrix samples were annealed as quickly as possible, and more spectra
were collected; selected samples were subject to broadband photolysis
by a medium-pressure mercury arc (Philips, 175 W) with globe removed
(240-580 nm).
Figure 2. Infrared spectra in the 930-830 cm-1 region for laser-ablated
scandium atoms deposited with 2% N2 (14N2/14N15N/15N2 ) 1/2/1) in
argon: (a) after deposition for 1.5 h at 10 K; (b) after annealing to 25
K; (c) after annealing to 30 K; (d) after annealing to 35K.
Results
FTIR spectra of the Sc + N2 reaction products in solid Ar
and N2 and DFT calculations on product molecules will be
presented. One feature of these systems is that some bands in
argon matrices are almost unshifted from their nitrogen matrix
counterparts. Nitrogen isotopic substitution was employed for
band identification; isotopic shifts will be discussed in the next
section together with band assignments. All systems revealed
weak bands for scandium monoxides and dioxides and com-
plexes of monoxides with nitrogen owing to oxide contamination
on the scandium target surface.12 In solid nitrogen, the strong
1657.7 cm-1 N3 radical band was observed.24,25
Sc + N2 Diluted in Ar. Product absorptions for Sc + N2 in
argon are shown in Figures 1-3 and are listed in Table 1.
Deposition revealed important absorptions at 2304.9, 2160.4,
2004.1, 1902.0, 1833.8, 1807.3, 1714.0, and 1599.1, in the upper
region, which is shown in Figure 1. Significant absorptions
appear at 913.0, 898.4, 807.3, 772.2/769.1, and 672.9/668.2
cm-1 in the lower region. Figure 2 shows spectra for reaction
products in the 930-830 cm-1 region using a statistically
scrambled 14N2:14N15N:15N2 sample, and the 14N2 reaction
product spectrum from 820 to 620 cm-1 is illustrated in Figure
3. Annealing decreased the 1902.0 cm-1 band, increased all
other bands, and produced new 1833.8/1807.3 and 1572.7 cm-1
absorptions in the high-frequency region. Annealing first
Figure 3. Infrared spectra in the 820-620 cm-1 region for laser-ablated
scandium atoms deposited with 2% N2 in argon: (a) after deposition
for 1 h at 10 K; (b) after annealing to 25 K; (c) after annealing to 30
K; and (d) after annealing to 40 K.
increased the sharp 913.0 cm-1 band, then the 898.4 cm-1 band,
and finally the broader 865.0 cm-1 band. These absorptions
gave nitrogen isotopic doublets with the scrambled reagent in
Figure 2. The doublets at 772.2/769.1 and 672.9/668.2 cm-1
increased on annealing to 25K and decreased on annealing to
(21) Andrews, L.; Chertihin, G. V.; Ricca, A.; Bauschlicher, C. W., Jr.
J. Am. Chem. Soc. 1996, 118, 467.
(22) Thompson, C. A.; Andrews, L. J. Am. Chem. Soc. 1996, 118, 10242.
(23) Lanzisera, D. V.; Andrews, L. J. Am. Chem. Soc. 1997, 119, 6392.
(24) Tiam, R.; Facelli, J. C.; Michl, J. J. Phys. Chem. 1988, 92, 4073
(25) Hassanzadeh, P.; Andrews, L. J. Phys. Chem. 1992, 96, 9177.

Reactions of Laser-Ablated Scandium Atoms
J. Am. Chem. Soc., Vol. 120, No. 13, 1998 3207
Table 1. Absorptions (cm-1) Observed in Reaction of Scandium
Atoms with Nitrogen on Condensation with Excess Argon at 10 K
14N2
15N2
R (14/15) anneal
assignment
(NN)x(ScN)2
(NN)xSc(N2)
(NN)xSc(N2)
Sc(NN)x
Sc(NN)x
ScNN
(NN)xSc(N2)
(Nx)Sc(N2)
Sc(N2)?
Sc(N2)2Sc
Sc(N2)2Sc
ScO blue site
ScO red site
ScN
(NN)xScN
(NN)xScN
2304.9a 2227.8 1.0346
2182.9 2110.5 1.0343
2160.4 2089.0 1.0342
++
+
+
+
+
-
+
+
+
+
+
2017
1950
1.0343
2004.1 1937.8 1.0342
1902.0b 1839.0 1.0343
1833.8 1772.7 1.0345
1807
1748
1.0347
1714.0 1657.2 1.0343
1599.5 1547.6 1.0335
1572.7 1521.2 1.0340
976.3
954.8
913.0c
898.4
865.0c
807.3d
805.2
803.3
772.2e
769.1e
766.0e
763.1
759.7
756.4
753.0
749.2
745.1
740.3e
735.9e
722.3
672.9e
668.2e
664.1e
659.9
655.6
654.0
652.2
648.9e
641.3e
590.5c
976.3
954.8
889.7 1.0262
875.5 1.0262
843.2 1.0261
787.0 1.0258
785.0 1.0257
783.3 1.0255
752.6 1.0260
749.5 1.0262
776.4 1.0263
743.4 1.0265
740.1 1.0265
737.4 1.0258
733.4 1.0267
729.3 1.0273
725.6 1.0269
721.4 1.0262
717.0 1.0264
1.0257
656.0 1.0258
651.3 1.0259
647.2 1.0261
643.0 1.02631
638.9 1.0261
637.0 1.0264
635.4 1.0264
632.2 1.0264
625.1 1.0259
575.4 1.0262
-
+
+
-
(ScN)2+ isolated (b2u)
(ScN)2+ plus NN
(ScN)2+ plus NN
(ScN)2 isolated (b2u)
(ScN)2 plus NN
(ScN)2 plus NN
(ScN)2 plus NN
(ScN)2 plus NN
(ScN)2 plus NN
(ScN)2 plus NN
(ScN)2 plus NN
(ScN)2 plus NN
(ScN)2(NN)x saturated (b2u)
(ScN)2(NN)x site
OScO
(ScN)2 isolated (b3u)
(ScN)2 plus NN
(ScN)2 plus NN
(ScN)2 plus NN
(ScN)2 plus NN
(ScN)2 plus NN
(ScN)2 plus NN
(ScN)2 plus NN
(ScN)2(NN)x saturated (b3u)
?
-
-
+,-
+,-
+,-
+,-
+
-
-
+
Figure 4. Infrared spectra in the 800-600 cm-1 region for laser-ablated
scandium atoms with 2% mixed isotopic nitrogen in argon: (a) 1%
14N2 + 1% 15N2, after deposition for 1 h; (b) sample a, after annealing
to 25 K; (c) sample b after annealing to 40 K; (d) 0.5% 14N2 + 1%
14N15N + 0.5% 15N2 after deposition for 1.5 h and annealing to 35K.
+
++
++
+
+,-
+,-
+,-
+,-
+
same proportion (Figure 5). The 1699.4 cm-1 absorption in-
creased then decreased on the annealing sequence. Annealing
also produced sharp weaker bands at 921.8 and 832.7 cm-1
.
+
The 1759.5 cm-1 band and 1762.2 and 1754.4 cm-1 satellites
increased on annealing and photolysis, while the 864.8 cm-1
absorption increased on annealing and decreased on photolysis,
which increased a 857.4 cm-1 feature. We emphasize here that
the 2300, 864.8, 740.3, and 641.2 cm-1 bands almost coincide
with respective argon matrix counterparts and the 1699.4 cm-1
band is just 14.6 cm-1 below the 1714.0 cm-1 argon matrix
band.
+
+
++
-
a Doublet with 14N2 + 15N2 and triplet 2304.9/2266.6/2227.8 cm-1
with 14N2 + 14N15N + 15N2. b Doublet with 14N2 + 15N2 and quartet
1902.0/1872.0/1869.4/1839.0 cm-1 with 14N2
+
14N15N + 15N2.
c Doublets with both mixtures. d Doublets with 14N2 + 15N2 and triplets
807.3/797.3/787.0 and 805.2/795.1/785.0 cm-1 with 14N2 + 14N15N +
15N2. e Doublets with 14N2 + 15N2 and triplets with 14N2 + 14N15N +
15N2 at 772.2/762.7/752.6, 769.1/759.8/749.5, 766.0/756.3/746.4, 740.3/
731.0/721.4, 735.9/726.4/717.0, 672.9/663.0/656.0, 668.2/658.3/651.3,
Figure 6c,d shows the spectrum from a mixture of 14N2 and
15N2. Most absorptions contained only the two pure isotopic
components, but the 2177.6 and 1759.5 cm-1 bands revealed
new intermediate (i.e. mixed) components and the 739.2 and
641.2 cm-1 band systems exhibited weak intermediate isotopic
counterparts at 729.9 and 631.6 cm-1. Note that, on annealing,
the latter mixed isotopic bands increased relative to the pure
isotopic bands. Figure 6a,b shows the analogous spectrum from
664.1/654.1/647.2, 641.3/631.3/625.4 cm-1
.
higher temperatures while 740.3/735.9 and 641.3 cm-1 bands
appeared in the spectra (Figure 3). On annealing intensities of
the 772.2/769.1 and 672.9/668.2 cm-1 absorptions are changed
in the same proportion; the new 2304.9, 740.3/735.9, and
641.3 cm-1 bands also showed the same correlation. Spectra
with mixed 14N2 + 15N2 samples gave doublets, i.e., the sum of
pure isotopic spectra, but reactions with statistically scrambled
14N2 + 14N15N + 15N2 gave triplets with new mixed interme-
diate components as listed in Table 1 (Figure 4). In other
experiments annealing to 25 K increased the 772.1/769.1 and
672.9/668.2 cm-1 bands by 50% and produced the 740.3 and
641.3 cm-1 bands and decreased the 807.3/805.2 cm-1 bands
by 30%.
a statistical 14N2
+
14N15N + 15N2 sample. Note that the
intermediate components are now stronger than the pure isotopic
components, which is more clearly seen after annealing.
DFT Calculations. Density functional theory calculations
were performed for small scandium nitride molecules using the
BP86 functional in the Gaussian 94 program system employed
for scandium oxides.12,26 The 6-31+G* basis set is used for
(26) Gaussian 94, Revision B.1. Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.;
Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.;
Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.;
Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng,
C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E.
S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.;
Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople J. A.
(Gaussian, Inc., Pittsburgh, PA, 1995).
Sc + N2. In solid nitrogen, weak 2300 cm-1, strong 2177.6
cm-1, broad 2003 cm-1, and strong 1699.4 cm-1 bands and
weak 1759.5, 864.8, 740.3, and 641.2 cm-1 absorptions were
observed (Table 2). The 2300 cm-1 band system and the 739.2
and 641.2 cm-1 bands markedly increased on annealing in the

3208 J. Am. Chem. Soc., Vol. 120, No. 13, 1998
Chertihin et al.
Table 2. Absorptions (cm-1) Observed in Reaction of Scandium Atoms with Nitrogen on Condensation with Excess Nitrogen at 10K
14N2
15N2
14N2+14N15N+15N2
R (14/15)
anneal
assignment
2327.6
2306.6
2300.8
2298.6
2294.9
2291.0
2177.6
2114.8
2112.0
2026
2249.8
2229.6
2223.6
2221.0
2218.4
2214.6
2105.0
2045.0
2042.6
1959
1.0346
1.0345
1.0347
1.0345
1.0345
1.0345
1.0345
1.0341
1.0340
1.034
-
N2
2306.6,2267.4,2229.6
2300.8,2262.5,2223.6
2298.6,2260.7,2222.0
2295.4,2254.9,2218.3
2289.5,2253.3,2213.4
2177, 2156, 2142, 2121, 2106
+
(ScN)2(NN)x
(NN)xScN
(NN)xScN
(NN)XScN
site
(NN)x Sc(N2)
?
+
+
+
+
+
+, -
+, -
+
?
Sc(NN)x
Sc(NN)x
Sc(NN)x
Sc(NN)x
Sc(NN)x
Sc(NN)x
(N2)Sc(N2)
(N2)Sc(N2)
(N2)Sc(N2)
(NN)xSc(N2)
(NN)xSc(N2)
(NN)xSc(N2)
(NN)xSc(N2)
N3
((NN)xSc2N)
(NN)xScN
(NN)xScN
(NN)xScN
(NN)xScN
(NN)xScN
?
2004
1938
1.034
+
1966.1
1955.2
1925.3
1907.0
1762.2
1759.5
1757.4
3373.7
3368.5
1701.2
1699.4
1657.7
921.8
868.7
864.8
864.2
863.1
857.4
832.7
812.8
811.8
1900.7
1890.3
1861.4
1844.1
1703.8
1701.2
1699.2
3261.8
3258.2
1644.5
1643.4
1603.3
895.2
846.4
842.6
842.1
840.9
835.4
813.8
792.5
791.1
1.0344
1.0343
1.0343
1.0341
1.0343
1.0343
1.0343
1.0340
1.0339
1.0345
1.0341
1.0339
1.0297
1.0263
1.0263
1.0262
1.0264
1.0262
1.0232
1.0256
1.0262
1.0264
1.0262
1.0262
1.0262
+, -
+, -
+, -
+, -
+
1744.5,1733.4, 1721.1,1715.2
1741.7,1730.6, 1718.5,1712.5
1739.7,1728.6,1716.3,1710.6
3371.8,3318.0,3262.5
+
+
+,-
+,-
+,-
+,-
-
1699.0,1671.6,1643.6
921.8,895.2
+
+,-
+
864.9,842.7
864.2,842.0
863.1,840.9
857.3,835.4
+
+
-
833, 826, 817, 814
+
+
? (x)ScN
? (x)ScN
(ScN)2(NN)x (b2u)
site
+
739.2
738.5
645.6
641.2
720.3
719.5
629.1
624.8
739.2,729.9,720.3
738.2,728.5,717.2
645.2,635.2,628.8
641.0,631.6,624.8
+
+
+
site
+
(ScN)2(NN)x (b3u)
nitrogen.27 For Sc [8s 4p 3d] contraction of the (14s 9p 5d)
primitive set developed by Wachters was employed.28 The s
and p spaces are contracted using contraction number 3, while
the d space is contracted (311). To this basis set one diffuse p
function with exponent 0.13462 is added; this is the tight
function optimized by Wachters multiplied by 1.5. The second,
more diffuse, p function is not included as it was found to affect
results very slightly, but introduced severe convergence prob-
lems to the orbital optimization procedure. The Hay diffuse d
function with exponent 0.0588 is also added.29
First, ScN was calculated to have the 1Σ+ ground state
with 1.678 Å bond length and 930 cm-1 harmonic frequency.
For this state MCSCF calculations obtained 748 cm-1 and
1.768 Å and later MCSCF/MRCI calculations revealed 774
cm-1 and 1.762 Å values.10 Rotational analysis provided a
1.687 Å bond length.7 We will show below that the ScN
frequency in solid argon is 913 cm-1, which we suspect is
within 10 cm-1 of the gas phase value. Clearly DFT/BP86
calculations give a better representation of the ScN molecule
bond length and vibrational frequency. Considerable success
in predicting vibrational frequencies has been found recently
for DFT/BP86 calculations with beryllium23 and transition metal
systems.12,30-32
The singlet complex (NN)ScN is bound by 13.3 kcal/mol and
the Sc-N stretching fundamental is 911 cm-1
The next
.
complex in the series, (NN)2ScN, is bound by 25.3 kcal/mol
and the Sc-N fundamental is 882 cm-1. Note (Table 3) that
the infrared intensity of the Sc-N fundamental increases with
ligation by dinitrogen at scandium.
1
The rhombic dimer (ScN)2 has a Ag ground state and is
bound by 142 kcal/mol with respect to two ScN molecules.
Calculated frequencies are given in Table 3. The 3B2u state of
2
+
(ScN)2 and the B2u state of (ScN)2 are 29.2 and 144.3 kcal/
mol higher, respectively.
Three ScN2 species were calculated: the linear ScNN quartet
is the global minimum with the side-bound quartet 2.5 kcal/
mol higher, and the linear doublet NScN is 92 kcal/mol higher.
The diagnostic N-N stretching modes were calculated at 1870
and 1744 cm-1, respectively, for ScNN and Sc(N2). Three Sc-
(N2)2 species were considered: the linear quartet is the most
stable, more stable than N2 + ScNN by 20.1 kcal/mol.
The Sc 3d14s2 occupation is lower than the 3d24s1 occupa-
tion, but the doubly occupied 4s orbital results in a much
larger Sc-NN repulsion for the 3d14s2 occupation than for the
3d24s1 occupation. This leads to a longer Sc-NN distance for
the 3d14s2 occupation. The d to π* donation is natually smaller
for the 3d14s2 occupation because of the longer Sc-N distance
and because there is only one d electron available to donate to
N2. Thus promotion to 3d24s1 is favored for ScNN and Sc-
(27) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80,
3265 and references therein.
(28) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033.
(29) Hay, P. J. J. Chem. Phys. 1977, 66, 4377.
(30) Fournier, R. J. Chem. Phys. 1993, 99, 1801.
(31) Jones, V.; Thiel, W. J. Chem. Phys. 1995, 102, 8474.
(32) Rosi, M.; Bauschlicher, C. W., Jr.; Chertihin, G. V.; Andrews, L.
Theor. Chem. Acc. 1998. In press.
4
(N2). The bonding in the Σ- state of ScNN arises from the
3d24s1 occupation of Sc. The dπ orbital donates charge to the
N2 π* orbital and the N2 donates charge to the Sc in the σ space.
The 4s orbital mixes with both the Sc 4p and 3d σ orbitals to

Reactions of Laser-Ablated Scandium Atoms
J. Am. Chem. Soc., Vol. 120, No. 13, 1998 3209
Table 3. Harmonic Frequencies and Bond Distances Calculated
for Scandium Nitrides Using DFT/BP86
frequencies, cm-1
(intensities, km/mol)
molecule
ScN, 1Σ
distances, Å
1.678
930 (10)
(NN)ScN
1.130, 2.259, 911 (48), 2188 (503)
1.703
(NN)2ScN
1.128, 2.279, 882 (71), 2190 (848),
1.723
2213 (210)
ScNN,4Σ-
2.015, 1.167 1870 (688), 412 (14), 260 (2)
E ) 0.0a
Sc(N2), 4B1
E ) +2.5
2.147, 1.195 1744 (376), 363 (2), 350 (18)
NScN, 2Πu
1.763
1.895
1.915
1.900
662 (0), 359 (63), 132 (213)
E ) +92
(ScN)2, 1Ag
E ) 0.0a
792 (176), 735 (0), 651 (223),
474 (0), 431 (0), 254 (112)
701 (0), 672 (1797), 527 (52),
525 (0), 363 (0), 54 (30)
838 (220), 730 (0), 542 (64),
521 (0), 385 (0), 204 (151)
(ScN)2, 3B2u
E ) +29.2
(ScN)2+, 2B2u
E ) +144.3
Sc(NN)2, 4Σg- linear
E ) 0.0a
2.114, 1.151 2043 (0), 1961 (2720),
375 (2), 367 (91)
(NN)Sc(N2), 4A′′, planar
E ) +5.6
2026 (687), 1859 (786),
390 (0.1), 353 (1)
Sc(N2)2, 4B1, D2d
E ) +5.8
2.199, 1.193 1794 (0), 1756 (1239),
377 (13), 328 (40)
a Relative energies of structural isomers in kcal/mol.
Figure 5. Infrared spectra in the 2350-1650 and 880-620 cm-1
regions for laser-ablated Sc atoms deposited with pure nitrogen: (a)
deposition for 2 h at 10 K; (b) after annealing to 25 K; (c) sample b
after annealing to 30 K; (d) sample c after annealing to 40 K.
the Sc, only sd σ hybridization occurs for NNScNN. Thus the
4s population is smaller for NNScNN than ScNN (0.98 vs 1.18
electrons). The bonding mechanism is very similar for the 4B1
state of Sc(N2). The N2 π* orbitals are of b2 and a2 symmetry,
and therefore these are the two symmetries that contain the open-
shell d orbitals. The third open-shell is natually the Sc 4s(a1)
orbital.
The bonding in the 2Πu state of NScN is very different from
that in Sc(N2) and ScNN. The valence occupation, ie that
without the N 1s and 2s orbitals and the Sc 1s-3p orbitals, is
(σg)2(πg)4(πu)3. That is, N 2pσ and Sc 3dσ orbitals form a three-
center-two-electron bond. The N 2pπ and Sc 3dπ orbitals form
similar three-center bonds. The πu orbitals are essentially
localized on the N atoms. This bonding mechanism implies
that Sc is 3d3, which is very high in energy. However the Sc
donates almost an entire electron to the two N atoms. The
doublet state arsing from 4s4p hybridization of Sc, yielding a
(σg)2 (σu)2(πg)4(πu)1 occupation, is higher in energy, because
the optimal bond distance for the Sc 4s and 3d orbitals is
different. Thus this occupation yields two good σ bonds, but
2
the π bonding is weak and hence is above the Πu state with
the (σg)2(πg)4(πu)3 occupation. We should note that we have
2
found linear quartet states below the Πu state, but these have
imaginary frequencies.
Discussion
Infrared spectra of scandium nitrides will be assigned with
the help of isotopic substitution and DFT calculations.
ScN. The sharp weak band at 913.0 cm-1 in solid argon
containing 2% N2 increased then decreased on annealing cycles,
giving way first to a band at 898.4 cm-1 and then finally to a
broader band at 865.0 cm-1 (Figure 2). These bands exhibited
one nitrogen-15 component and 14/15 isotopic ratios 1.0262 (
0.0001, which is just under the harmonic diatomic value (1.0264)
and appropriate for the Sc-N fundamental vibration with small
cubic anharmonicity. In solid nitrogen an 864.8 cm-1 band
increased in absorbance and structure on annealing in parallel
with a set of bands near 2300 cm-1 (Figure 4). The former
Figure 6. Infrared spectra in the 880-600 cm-1 region for lasser-
ablated Sc atoms deposited with mixed isotopic nitrogen: (a) 14N2/
14N15N/15N2 ) 1/2/1 sample deposited for 1 h; (b) sample a after
annealing to 35 K; (c) 14N2/15N2 ) 1/1 sample deposited for 1.5 h; (d)
sample c after annealing to 35 K.
polarize away from the N2 and hence reduce the Sc-NN
repulsion.
Bonding in the 4Σg- state of NNScNN is very similar to that
in the 4Σ- state of ScNN. With N2 molecules on both sides of

3210 J. Am. Chem. Soc., Vol. 120, No. 13, 1998
Chertihin et al.
band showed a single nitrogen-15 counterpart at 842.6 cm-1
with the 14/15 ratio of 1.0263 and the latter band gave triplets
with scrambled 14,15N2 (Table 2) with the 14/15 ratio of 1.0346
( 0.0001. The former band is clearly due to an Sc-N vibration
and the latter band is due to a dinitrogen vibration. These bands
are appropriate for a (NN)xScN complex where x could, in
principle, be as large as six. The 898.4 cm-1 band in solid
argon containing 2% N2 is due to an incompletely saturated
(NN)xScN complex (x < 6). Note agreement between the much
weaker 865.0 cm-1 band formed on annealing in solid argon
and the stronger 864.8 cm-1 band in solid nitrogen for the (NN)x-
ScN species. Photolysis decreased the 864.8 cm-1 band in favor
of a sharp new 857.4 cm-1 feature, but further annealing
increased the 864.8 cm-1 band and decreased the 857.4 cm-1
band.
The 913.0 cm-1 band in solid argon is assigned to isolated
ScN. This band is in excellent agreement with the 930 cm-1
DFT/BP86 calculation but substantially higher than the 774
cm-1 MCSCF/MRCI value10 and the 795 cm-1 frequency
predicted from empirical correlation with bond length.7 Fur-
thermore, the excellent agreement between the DFT/BP86 bond
length and the gas-phase value7 bodes well for the BP86
prediction of fundamental frequency. On the basis of agreement
between the gas phase (1039.6 cm-1) and argon matrix (1032.4
cm-1) fundamentals1,15 for TiN and the gas phase (1020 ( 5
cm-1) and argon matrix (1026.2 cm-1) fundamentals3,14 for VN,
the gas-phase fundamental for ScN is predicted to be in the
920-910 cm-1 range.
(Figure 4) and exhibit 14/15 ratios similar to the argon matrix
counterparts.
The two lower bands in both solid argon containing 2% N2
and pure nitrogen give triplet absorptions with statistical 14,15N2
clearly identifying vibrations of two equivalent N atoms. The
sharp 772.2 and 672.9 cm-1 bands are assigned to the b2u and
b3u stretching modes of the isolated rhombic molecule (ScN)2,
which requires the ScN diatomic 14/15 isotopic ratio. These
assignments are substantiated by the DFT calculations, which
predict strong b2u and b3u modes at 792 and 651 cm-1 for the
1Ag ground state, in excellent agreement with the observed
values.
The strong 2304.9, 740.3, and 641.3 cm-1 bands that appear
on annealing in solid argon are due to the N2-ligated (ScN)2
species denoted (ScN)2(NN)x with dinitrogen ligands bonded
to both metal centers. Although the 2304.9 cm-1 band gives a
triplet absorption with 14,15N2, the dinitrogen ligands could be
end-bonded. The series of eight resolved bands leading from
772.2 to 740.3 cm-1 and seven resolved bands from 672.9 to
641.3 cm-1 are best explained by successively increasing num-
bers of dinitrogen ligands at each Sc position in the (ScN)2 ring.
We must note that if each Sc atom can attain 6-fold coordination,
a total of eight dinitrogen molecules will be required for satura-
tion of (ScN)2 to (ScN)2(NN)x. The nitrogen matrix spectrum
is clearly dominated by the saturated ligated species with nearly
the same band positions as the annealed argon matrix.
The rhombic (MN)2 species is also important in the titanium
system,15 but not for the V, Cr, Mn, and Fe reactions with
nitrogen.13,14 The (MN)2 rhombic dimer also appears with Co
and Ni.16 This derives from several factors including the overall
yield of MN produced, the yield of MNN intermediate, and the
stability of the ligated (NN)xMN complexes.
The weaker structured 807.3/805.2/803.3 cm-1 band system
(Figure 3) shows isotopic character just like the stronger 772.2
cm-1 band system, but the bands decrease about 30% on 25 K
annealing and another 30% on broad-band photolysis, whereas
the 772.2 cm-1 band increases slightly on 25 K annealing.
Similar DFT calculations predict the strongest (b2u) fundamental
of the (ScN)2+ species to appear 46 cm-1 higher than this mode
for (ScN)2, and the weak 807.3 cm-1 band is 35 cm-1 above
the strong 772.2 cm-1 band. Accordingly, the weaker 807.3
cm-1 band is probably due to (ScN)2+. The much weaker b3u
mode is predicted in the low 500 cm-1 region and is not
observed here. The 590.5 cm-1 band shows opposite photolysis
behavior and cannot be identified.
A similar situation exists for the CrN molecule where DFT/
BPL predicts a 1017 cm-1 fundamental for the 4Σ- ground state,
substantially higher than the 854 cm-1 MCSCF value but in
excellent agreement with the recent 1044 cm-1 argon matrix
and approximate 1000 ( 100 cm-1 gas-phase fundamen-
tals.5,10,14 Likewise the DFT/BPL predicted 1.555 Å bond length
for CrN is in much better agreement with the recent 1.563 Å
measurement from rotational analysis than the 1.619 Å MCSCF
value.5,10,14 In contrast the DFT and MCSCF calculations both
predict frequencies for TiN and VN in very good agreement
with gas phase and matrix determinations.1-3,10,14,15
Similar behavior has been observed for TiN, VN, CrN and
their dinitrogen complexes.14,15 In the case of VN, five distinct
(NN)xVN complexes were observed culminating in the most
extensively ligated species absorbing at 997.0 cm-1 in solid
nitrogen and 997.8 cm-1 after annealing in solid argon.
DFT calculations done to model the dinitrogen complexes
for (NN)ScN and (NN)2ScN show that the Sc-N fundamental
red-shifts and gains intensity with ligation in agreement with
experiment. The N-N fundamentals predicted near 2190 cm-1
are observed near 2300 cm-1 perhaps because the complexes
are more extensively ligated than the model compounds.
Work in progress with Sc and Ar/H2 samples containing a
trace of N2 impurity (<0.1%) deposited at 7 K gives a low yield
of the 807.3, 772.2, and 672.9 cm-1 bands with only weak 769.1
and 668.2 cm-1 satellites. Annealing to 23 K doubles the 772.2
and 672.9 cm-1 bands, triples the 769.1 and 668.2 cm-1
satellites, and leaves the 807.3 cm-1 band unchanged. Final
annealing to 38 K decreases the 807.3 cm-1 band (to 40%) and
the 772.2 and 672.9 cm-1 bands (to 70%), slightly increases
only the first three (labeled plus NN) satellites, and does not
produce the 740.3 and 641.3 cm-1 bands. These recent
observations provide further support for the above conclusions.
We note that the present 913.0 cm-1 assignment for ScN in
solid argon is near the ScO argon matrix (955 cm-1 12
) and gas
phase (965 cm-1) values just as the TiN fundamental is near
the TiO fundamental (argon matrix, 988 cm-1 and gas phase,
1000 cm-1).33,34
(ScN)2. The argon matrix spectra are characterized by strong,
sharp band series starting at 772.2 and 672.9 cm-1 that first
increase and then decrease on annealing cycles in favor of strong
bands at 740.3 and 641.3 cm-1, which are accompanied by a
strong 2304.9 cm-1 band. The former bands exhibit ScN
diatomic isotopic ratios whereas the latter band shows the ratio
for dinitrogen. In the nitrogen matrix, weak analogous 2306.6,
739.2, and 641.2 cm-1 bands increase together on annealing
ScN2. DFT calculations predict that quartet linear ScNN is
only slightly more stable than cyclic Sc(N2); these molecules
have strong absorptions calculated at 1870 and 1744 cm-1
,
respectively. Examination of the argon and nitrogen matrix
spectra in this region reveals sharp bands at 1902.0 and 1714.0
cm-1 in solid argon and a strong band at 1699.3 cm-1 with an
associated band at 2177.6 cm-1 in solid nitrogen. The sharp
1902.0 cm-1 argon matrix band becomes a 1902.0/1872.0/

Reactions of Laser-Ablated Scandium Atoms
J. Am. Chem. Soc., Vol. 120, No. 13, 1998 3211
1869.4/1839.0 cm-1 quartet with statistical 14,15N2, which
indicates two inequivalent nitrogen atoms, but the 1714.0 cm-1
band was too weak to observe mixed components. However,
the strong 1699.3 cm-1 nitrogen matrix band gave a 1699.0/
1671.6/1643.6 cm-1 triplet, for two equivalent nitrogen atoms,
and the associated 2177.6 cm-1 band gave a broad pentet with
strong median peak using statistical 14,15N2. Clearly, the former
band involves two equivalent nitrogen atoms, i.e., sideways
bonded N2 and the latter band involves a number of end-bound
NN subunits.
becomes a sharp unshifted doublet with mixed isotopic precur-
sors and exhibits a higher 14/15 isotopic ratio (1.0297) than
the ScN diatomic value. This suggests that a single N is
vibrating against a heavier mass than one Sc atom. Accordingly,
the weak 921.8 cm-1 band is tentatively assigned to Sc2N, which
must be complexed with dinitrogen as (NN)xSc2N. The 832.7
cm-1 band exhibits a 14/15 isotopic ratio (1.0232) and inter-
mediate isotopic components for an aggregate species that
cannot be identified without more information.
Reaction Mechanisms. The important step in these reactions
of laser-ablated Sc atoms is dissociation of molecular nitrogen
to atoms, and this is caused by the impact with energetic Sc
atoms from laser ablation.13,14,37 Similar ablation methods have
produced Sc atoms with kinetic energies up to 30 eV.38 ScN
is formed in these experiments by direct atom combination,
reaction 1. A similar mechanism was proposed for the
Accordingly the 1902.0 cm-1 band is assigned to ScNN in
solid argon. The 2177.6 and 1699.4 cm-1 bands are assigned,
respectively, to end-bound NN ligands and the side-bound N2
complex of (NN)xSc(N2) in solid nitrogen. The weak 1714.0
cm-1 band is probably the argon matrix counterpart, Sc(N2),
but we cannot be certain. The bands at 1833.8 and 1807 cm-1
that grow on annealing appear to be due to partially coordinated
(NN)xSc(N2) species. The major strong band at 2004 cm-1 in
both matrixes is probably due to the coordinatively saturated
Sc(NN)x species with x likely to be 6.
Sc + N f ScN
(1)
production of FeN.13 The observation of N3 radical attests to
the formation of N atoms in these experiments.24,25
The 1699.4 cm-1 band reveals splittings on annealing at
1701.2 and 1704.3 cm-1 (Figure 5), which are presumably due
to different orientations of the (NN)x ligands or the entire
(NN)xSc(N2) molecule within the matrix cage; these splittings
are matched in behavior by a much weaker band at 3368.5 cm-1
(1.3% of the absorbance at 1699.4 cm-1) with splittings at
3372.7 and 3378.5 cm-1. The latter bands are clearly due to
overtones of the former bands. This is confirmed by the con-
stant difference (29.9 ( 0.2 cm-1) between the overtone and
two times the fundamental and the matching isotopic ratios and
statistical mixed isotopic triplet patterns. It is interesting to note
that the 14N2 components for the fundamental and overtone are
slightly different in the statistical isotopic mixture because the
dinitrogen ligands are isotopically substituted as well. Such
overtone bands have been observed for other (NN)xM(N2)
species where the product yield was relatively high including
Y, Sm, Eu, and Yb; the overtone band may serve as an
additional diagnostic for the sideways bound M(N2) species.35,36
Other Absorptions. Two other absorptions show evidence
for two dinitrogen subunits. The matrix-split band at 1762.2,
1759.5, and 1757.4 cm-1 in solid nitrogen exhibits an interme-
diate feature at 1721.1, 1718.5, and 1716.3 cm-1 with 14N2 +
15N2 and three new intermediate matrix-split (Table 2) bands
using 14N2 + 14N15N + 15N2 with 1:4:6:2:4:x intensities relative
to the above 14N2 trio (the 15N2 trio was masked by the strong
1699.3 cm-1 (NN)xSc(14N2) band). DFT calculations predict
the linear NNScNN and D2d Sc(N2)2 species to have very strong
Clearly more N atoms are produced in the nitrogen matrix
experiments, hence the higher yield of ScN as ligated (NN)xScN
and the observation of N3 radical in solid nitrogen.
xN2 + Sc + N f (NN)xScN
(2)
The mechanism of formation of (ScN)2 is of interest as two
transition metal atoms effectively convert N2 to a nitride, which
may assist in understanding the important nitrogen fixation
process. In the argon matrix experiments with 14N2 + 15N2,
doublets are observed for the (ScN)2 bands, indicating that a
single N2 molecule is involved in the reaction, which must
proceed through an intermediate ScN2 species, either ScNN or
Sc(N2), as indicated in eq 3. Dimerization of ScN is not the
Sc + N2 + Sc f ScN2 + Sc f (ScN)2
(3)
route in solid argon, as the ScN yield is relatively low in argon
matrix experiments. Furthermore, the dimerization mechanism
would require a mixed isotopic (Sc14NSc15N) component, and
such is not observed. Even on annealing, there is a 50% growth
of (Sc14N)2 and (Sc15N)2 absorptions but no growth of a mixed
isotopic component (Figure 4), and reaction 3 produces more
(ScN)2 species on annealing in solid argon.
On deposition with pure 14N2 + 15N2 (Figure 5c), the major
products involve one dinitrogen reagent molecule according to
reaction 4. However, on annealing, notice the growth of pure
antisymmetric N-N stretching modes at 1961 and 1756 cm-1
,
respectively. Clearly, agreement with the latter is excellent and
the above matrix-split bands are assigned to Sc(N2)2. The
observed spectrum is textbook for a species with two equivalent
N2 subunits and equivalent atomic positions except that the third
site-split component is slightly more intense than expected. The
major yield of Sc(N2) in solid nitrogen makes the Sc(N2)2
species a likely minor product.
(NN)xSc(N2) + Sc f (ScN)2(NN)x
(4)
isotopic products (Sc14N2)(NN)x and (Sc15N)2(NN)x and the
mixed isotopic product (Figure 5d), indicating that reaction 4
and dimerization reaction 5 both contribute to the product yield
in solid nitrogen where the initial yield of (NN)xScN is high.39
The sharp 1599.5 cm-1 band in solid argon containing 2%
N2 has a broader 1572.7 cm-1 counterpart, which grows on
annealing, exhibits a 1548 cm-1 intermediate band in 14N2 +
15N2 experiments, and indicates two dinitrogen molecules. This
region is appropriate for (>NdN) bridged dinitrogen, and these
bands are tentatively assigned to the Sc(µ-N2)2Sc ring species.
Finally, sharp weaker bands appear on annealing at 921.8
and 832.7 cm-1 in solid nitrogen. The 921.8 cm-1 band
2 (NN)xScN f (ScN)2(NN)x
(5)
(35) Chertihin, G. V.; Bare, W. D.; Andrews, L. J. Phys. Chem. A In
press. Annealing to 35 K destroys the N3- trapped in solid nitrogen in laser-
ablated Y and La atom experiments.
(36) Willson, S. P.; Andrews, L. To be published.
(37) Kang, H.; Beauchamp, J. L. J. Phys. Chem. 1985, 89, 3364.
(38) Thiem, T. L.; Salter, R. H.; Gardner, J. A. Chem. Phys. Lett. 1994,
218, 309.
(33) Chertihin, G. V.; Andrews, L. J. Phys. Chem. 1995, 99, 6356.
(34) Merer, A. J. Annu. ReV. Phys. Chem. 1989, 40, 407.
(39) The product of (4) and (5) contains dinitrogen ligands on both Sc
centers and could be written as (NN)x(ScN)2(NN)x.

3212 J. Am. Chem. Soc., Vol. 120, No. 13, 1998
Chertihin et al.
The reaction of another Sc atom with (NN)xScN produces
the discandium mononitride species, reaction 6, which following
the example of ScN, must be ligated by dinitrogen in the
nitrogen matrix.
trapped, and the clear growth of (NN)xScN, (NN)xSc(N2),
(ScN)2(NN)x, and (ScN)2 on annealing argues strongly that
these are neutral species. Any scandium-nitrogen cationic
species are expected to be minor contributors to the observed
spectrum.
The weak 807.3 cm-1 band system that is almost destroyed
on annealing is probably due to (ScN)2+, which can be made
by reaction of Sc+ or by photoionization of (ScN)2 with radiation
from the laser plume.
(NN)xScN + Sc f (NN)xScNSc(NN)x
(6)
Are any of these N2 reactions possible with thermal Sc atoms?
Preliminary thermal Sc atom matrix experiments of Klotzbuch-
er40 with 10% N2 in argon reveal weak 2304, 744, and 641 cm-1
bands (A ) absorbance ) 0.02) that are in agreement with the
absorptions assigned here to (ScN)2(NN)x and the 1833 cm-1
band assigned here to (NN)xSc(N2) in solid argon. With pure
nitrogen, thermal experiments gave the 1699 cm-1 band for
(NN)xSc(N2) in solid nitrogen. However, no bands were
observed for ScN or (NN)xScN species with thermal Sc atoms
confirming the need for dissociation of N2 to form ScN, which
is provided by excess energy in the laser-ablated scandium
atoms. Thus, the (ScN)2 molecule can be made by reaction 3
or 4 using thermal Sc atoms.
Conclusions
Laser-ablated Sc atoms react with nitrogen to form ScN,
Sc(N2), and (ScN)2 and their N2 ligated counterparts. These
identifications are based on isotopic shifts and agreement with
DFT frequency calculations. It is noteworthy that annealed
argon matrix samples containing 2% N2 give almost the same
absorptions as nitrogen matrix samples for the ligated complexes
(NN)xScN and (ScN)2(NN)x. The rhombic (ScN)2 species is
made by reaction of a second Sc atom with an intermediate
ScN2 species and, thus, provides an interesting route to nitrogen
fixation with naked metal atoms.
What is the role of electrons or Sc+ ions from the laser-ablated
metal vapor plume in these experiments? The detection of N3
-
at 2003.3 cm-1 in similar Y and La experiments35 in solid
nitrogen shows that some charged species survive to reach the
matrix.41 Even if impact with Sc+ contributes to the dissociation
of N2, most of the metal cation species produced here are
probably neutralized during the condensation process. Anneal-
ing would almost certainly annihilate any charged species
Acknowledgment. We gratefully acknowledge the assistance
of W. D. Bare with the laser-ablation experiments and the
communication of unpublished thermal scandium experimental
results by W. Klotzbucher.
JA973701D
(41) The very strong Sc(NN)x band at 2004 cm-1 completely masks any
-
(40) Klotzbucher, W. E. Unpublished results.
N3 absorption present here.