W. D. CLOSSON AND HARRY B. GRAY
290 [CONTRIBUTION FROM
THE
Vol. 85
DEPARTMENT OF CHEMISTRY,COLUMBIA UNIVERSITY,NEWYORK27, N. Y.]
The Electronic Structures and Spectra of the Azide Ion and Alkyl Azides BY W. D. CLOSSON AND HARRY B. GRAY RECEIVED JULY 27, 1962 The electronic transitions of alkyl azides and hydrazoic acid have been assigned on the basis of molecular orbital theory and the electronic structure and energy levels in azide F n . The 2870 A. band in alkyl azides has been assigned to a ry rx* ( I & + -+ lAU) transition, and the 2160 A. band to an s-px + rY*( lZE+-P l A U )transition. The effects of solvent polarity and electronegative substituents upon the spectra of alkyl azides have been rationalized in terms of this model. .-)
Introduction The azide ion, N3-, is a linear triatomic molecule ion with sixteen valence electrons. The principal bands in the electronic spectra of a number of linear triatomic molecules isoelectronic with N3- have been assigned previously in terms of a molecular orbital energy level scheme.' I n the present paper the electronic spectrum of the N3- ion is analyzed in terms of molecular orbital theory. The observed energies of the excited states in N3then are used to assign the electronic spectra of several azido compounds. Molecular Orbital Theory for Linear Triatomic Molecules.-A linear triatomic molecule with D,h symmetry is illustrated in Fig. 1. The orbital transformation scheme for s and p valence orbitals is given in Table I. u-orbitals: *[Us+] *[Vu+]
= cM@(S) =
cbi@(Pz)
+ CL@[1/&(UI + + cL@[1/dz(gI -
UZ)] 0211
Here E is a single electron molecular orbital energy difference and 0 = f h H @ L d r . Singlet excited states due to a transition rg -+ ru* are 12u+,12u-and la,. Of these, only the lZg++ lZu+ transition is allowed. Electronic Spectrum of Na-.-The spectrum of N3- in water solution exhibits a weak band (E = 430) a t 43,500 ~ m . - l . ~This band is partly masked by a strong absorption which does not reach a maximum a t 53,000 cm.-l. These two bands are characteristic of linear triatomic molecules with sixteen valence electrons (COz,CS2)'and may be assigned as in (10) and (11)
-+--f
I&+
@[l/dZ(PXI
= *[1/%'%h
- PXJ - PY2)l
+ cL@P[1/&(Pxi + CM @X(PY)+ CL*II/V'~(PY~ +
= CX
*(*'[(nU)l
@M(Px)
(3)
u-orbitals :
(4) b 2 ) l
(5)
PYJI
(6)
The C's are subject to the usual conditions of normalization and orthogonality, and are different for ug+, uu+ and rU molecular orbitals. TABLE I ug
Pa
+
-
Zg
.. .. ..
T U
Px, PY
fl!z nu
-
[(.g+)bl2[2s112[2s21~[(.U
+)bI"(
X")b14[(Tg)14 = ' 28 +
(7)
orbital coefficients are approximately* CM(Tu) = c L ( T u ) = 1/45 (8) Thus we calculate 7ru
C[(T")*
(1) (a)
-(
cM@(S)
r-orbitals : *[?Txl *[Ty1
+
= cM@(pa)f
Cl*(Ol) cl@(Ul)
+ -
cZ@(02) c2@(02)
+ C*@(PX) + CZ@(PY)+ CI*(PY)
= CM@(P,)
= CM@(Py)
d l
(12) (13)
(14) (15)
Localized R-N orbital:
dm
@(s)
+ d2/3N P x )
- px)*(~y)'
The molecular orbital energy level scheme expected for such molecules is shown in Fig. 2. The single electron u-bonding molecular orbitals are considered more stable than the single electron a-bonding molecular orbitals. Azide Ion, Na-.-The azide ion is one of a number of important molecules and ions with sixteen valence electrons. The ground state of Na- is For Ns- the
=
- N(M)- Xcz)
(16)
The molecular orbital energy level scheme for RNg molecules is shown in Fig. 3. The relative energies of the single electron molecular orbitals are estimated using the level scheme for azide ion as a guide. For the fourteen valence electrons (sixteen minus the two in the ub R-N orbital) the ground state is
S
+
U"
*[UR]
\E[Ub]
\kRN[s-px]
M orbitals
Representation
(11)
(1) (2)
(R-N)(l) =
*'"'(rg)l
*(')/(Tu)]
lZU+ >53,000 ern.-'
(10)
Electronic Spectra of Azido Compounds.- The HNs and RN3 molecules are not linear, the R-N-(NN) angles being about A consideration of bond distances4 suggests that most of the a-bonding takes place between the middle and the end nitrogen. For such an arrangement of atoms the molecular orbitals are
n-orbitals: *"'[(?Tg)I
lU A 43,500 cin.-l
=
- &3(N,N)
(9)
A. D. Walsh, J. Chem. Soc., 2260 (1953); (b) R. S. Mulliken,
Con. J . Chem., 36, 10 (1958). (2) However, more exact orbital coefficients for PITS- are given by E. Clementi, J. Chem. Phys., 34, 1468 (1961).
(17)
The most stable singlet excited state involves the one electron excitation r y + rx*. For any reasonable value of A3, the second most stable singlet excited state is due to the s-pX+ry* transition. These excited states are related to the 'A, state in the N3- ion. In alkyl azides two electronic transitions are observed in the accessible portion of the ultraviolet region. These occur atnabout46,000 ern.-' (2160 A.) and 34,800 cm.-' (2870 A.), and are assigned to the s-px+ry* and aY+ax*transitions, respectively. In the limit of isolated R N and NN molecules AI = (1/3)[42~) - ~(2s)l A2 = -@(N,N) Aa = 0
(19) (20)
Assuming that the differences in the interelectronicrepulsion energies of the first excited states and the (3) A similar absorption band is found in the NaNa crystal. See S. K. Deb, ibid., 36, 2122 (1961). (4) L. Pauling and L. 0. Brockway, J. Am. Chcm. Soc., 69, 13 (1937).
ELECTRONIC STRUCTURE OF AZIDEION
Feb. 5, 1963
m
A
Y
Ya A
I I
I I
I I
YI
/
/
IL
/
IL
LL
x
XI Fig. 1.-Coordinate
/
/
/
291
x 1.
A
system for a linear triatomic molecule with D,h symmetry.
ground states for Na- and RNI are equal, the ry+rx* transition in RNs molecules is predicted a t (l/@)[AE(lZ,+
--f
lAU)] = 30,060 cm.-l
E
(21)
This is in reasonable agreement with the observed energy (A, is expected to be greater than -/3(N,N) due to theinteraction of p., in rx*). The s-px+~,* transition is predicted a t (1/d2)[AE(1Zg+ --f 'A")]
+ AI
(22)
For the nitrogen atom, Al may be estimated a t about 14,000 cm.-', using atomic spectral data compiled by Moore.6 Thus the s-px+ry* should appear around 44,000 cm.-l; the agreement between theory and experiment is quite satisfactory. Spectral Measurements.-The ultraviolet absorption spectra (from ca. 1950 to 3100 B.)of several representative alkyl azides, as well as that of hydrazoic acid, have been measured in a series of solvents of varying polarity. The empirical polarity constant "Z," proposed by Kosower,6 was used as a guide in choosing the solvents. The positions of the absorption maxima in the neighborhood of 2870 A. (34,800 cm.-I) of the various azides are presented in Table 11, and those of the maxima near 2160 A. (46,300 cm.-I) in Table 111. The short wave length band of hydrazoic acid in water is partialjy masked but a maximum is apparent a t about 2000 A. The absorption spectrum of sodium azide in basic aqueous osolution was measured over the range 1900 to 3100 A. No distinct maxima were observed, the intensity rising to large values in the short wave length region. A shou@er, however, indicative of a maximum near 2300 A. (Emax = 440) was observed. The most noticeable feature seen in the data of Tables I1 and I11 is the insensitivity of the position of the maxima of most alkyl azides to gross changes in solvent polarity (as indicated by the polarity constant " Z " ) . For example, the first transition (at ca. 2870 A.) of cyclohexyl azide varies over a range of only 21 A. (250 cm.-I). Indeed, the transition energy of the first absorption band in isooctane and tetrafluoropropanol, solvents of vastly different polarity, are identical for n-butyl azide, and almost so for t-butyl and cyclohexyl azide. The position of the higher energy maximum is also quite insensitive to the polarity of the medium. The range for simple alkyl azides is about 32 A. (700 cm.-I) but this is mainly due to the fact that the extremely polar solvent TFP (tetrafluoropropanol) appears to shift this transition to significantly shorter wave lengths. Water and 10 M lithium chloride in water also have a similarly large effect on 2-hydroxyethyl azide. Plots of transition energies vs. Z for the low energy (2870 k.) bands of cy(5) C. E. Moore, "Atomic Energy Levels," U. S. Natl. Bur. Standards Circular 467, 1949 and 1952. (6) (a) E. M. Kosower, J . A m . Ckcm. Soc., 80, 3253, 3261, 3267 (1958). (b) E. M. Kosower and G. Wu, ibid., 83, 3142 (1961).
Fig. 2.-MolecuIar orbital energy level scheme for a linear triatomic molecule containing atoms with s and p valence orbitals. N trnl
Molecular 0 )bitals
Orbitill
N(2)l
(RN)(i)
Orbitals
6,"
E
1
Fig. 3.-Molecular
orbital energy level scheme for KNa type molecules.
clohexyl and 2-hydroxyethyl azide are presented in Fig. 4, and similar plots for the high energy transitions of some of the alkyl azides are shown in Fig. 5. The introduction of electronegative groups or atoms into the aliphatic portion of alkyl azides is shown to shift both transitions t o higher energies. Hydrazoic acid is apparently the extreme case, its lower intensity maximum being shifted about 230 k., and the higher energy band apparently being moved about 160 A. (Hydrazoic acid in dilute hydrochloric acid solution exhibited a maximum at about 2000 A., e = 540.) Discussion The low intensity (e = 25) of the 2870 A. maxima of alkyl azides might tempt one to consider it as a typical
iv. D. CLOSSON AND HARRY B. GRAY
292
TABLE I1 THELOXGWAVELESGTHABSORPTIOXMAXIXAOF
-----
M LiClb-
7-10 Am,,
n-Butyl azide Cyclohexyl azide t-Butyl azide 2-Chloroethyl azide 2-Hydroxyethyl azide 2-Acetoxyethyl azide Ethyl azidoacetate Hydrazoic acid
\
(Yti.3)C amax
Amax
..
2860 2862 2869 2822 2836 2819 2786
212 26
..
..
..
..
..
..
27 22 24 26
..
..
..
.. ..
..
2828
23
. .
..
..
..
. . ..
--Acetonitrile(71.3)' hmsx
n-Butyl azide 2877 Cyclohexyl azide 2883 &Butyl azide 2890 2-Chloroethyl azide 2842 2-Hydroxyethyl azide 2859 2-Acetoxyethyl azide 2850 Ethyl azidoacetate 2820 .. Hydrazoic acid a All values are considered valid to at least &5-6 A. propanol. e Not calculated.
~
~
Xm,,
%ILX
..
.. ..
2603
43
--.
23 25
Xmax
emas
2874 2877 2884 2840 2853 2843 2815
24 25
B
27 19 23 23
27 20 24 24
7
--Isooctane-(60 l I c emax
23 2G
2872 29 2881 31 2890 27 2837 33 19 2840 26 21 2840 29 25 2813 29 e .. 2644 Solution of lithium chloride in water.
n + T* transition, similar to those of carbonyl cornpounds such as ketones and aldehydes and possibly to those of xanthate esters, alkyl nitrites, and nitro-
-Ethanol--(79.ti)C
..
Solvent---Chloroform-(63.2) Xmax
~
%I%,
2875 2878 2886 2840 2853 2845 2816
.. 22
2832 ..
emax
~
~.~
Amax
.. ..
AZIDES"
--lIethan
