Journal of Molecular Spectroscopy 222 (2003) 263–272
www.elsevier.com/locate/jms
The microwave spectrum of the C3 sugars: glyceraldehyde
and 1,3-dihydroxy-2-propanone and the dehydration
product 2-hydroxy-2-propen-1-al
F.J. Lovas,a,* R.D. Suenram,a D.F. Plusquellic,a and H. Møllendalb
a
Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
b
Department of Chemistry, University of Oslo, Oslo, Norway
Received 28 May 2003; in revised form 30 July 2003
Abstract
The triose sugars with empirical formula, C3 H6 O3 , consist of the aldehyde form, glyceraldehyde, and the ketone form, 1,3-dihydroxy-2-propanone. Recently, the simplest sugar, glycolaldehyde (CH2 OHCHO) was identified in the Sgr B2(N) molecular cloud
by Hollis et al. (Astrophys. J. (Lett.) 540 (2000) L107), providing the incentive to pursue the present study. The microwave spectra of
the triose sugars were obtained with the NIST Fourier-transform pulsed-nozzle microwave spectrometers equipped with heated
nozzles. A few tenths of a gram of solid sample was placed in the nozzle base, which was heated to between 105 and 135 °C and
pressurized with inert carrier gas. Broad spectral survey scans were carried out from 10 to 20 GHz for samples of both compounds.
In the glyceraldehyde sample study, three conformers of the parent species were identified as well as 1,3-dihydroxy-2-propanone. In
addition, three decomposition products were also identified: formic acid, trans-methyl glyoxal, and a previously experimentally
unknown compound: 2-hydroxy-2-propen-1-al. Ab initio calculations were carried out with the Gaussian 98 program at the MP2/6311++G** level to aid in the identification of each of the new species. The survey scan of the 1,3-dihydroxy-2-propanone sample
confirmed the identification of this species initially assigned in the glyceraldehyde study. The 1,3-dihydroxy-2-propanone survey also
exhibited spectra from the same three decomposition products initially observed in the glyceraldehyde work. However, the three
conformers of glyceraldehyde were not present in the spectra of 1,3-dihydroxy-2-propanone.
Published by Elsevier Inc.
Keywords: Ab initio calculations; 1,3-Dihydroxy propanone; Dipole moments; Glyceraldehyde; 2-Hydroxy propenal; Microwave spectra; Molecular
structure
1. Introduction
The simple monosaccharide sugars are carbohydrates
with the empirical formula Cn H2n On , whereby glycolaldehyde, CH2 OHCHO, is represented by n ¼ 2 (a diose)
and glyceraldehyde, CH2 OHCHOHCHO, by n ¼ 3 (a
triose). When n P 3, a ketone form of the sugar also
exists. For the n ¼ 3 case, the ketone form is 1,3-dihydroxy-2-propanone, (CH2 OH)2 C @ O (or dihydroxyacetone). Glycolaldehyde was recently detected in the
interstellar cloud Sgr B2(N-LMH) by Hollis et al. [1].
This study provided the motivation to carry out a
*
Corresponding author. Fax: 1-301-869-5700/301-975-2950.
E-mail address: lovas@nist.gov (F.J. Lovas).
0022-2852/$ - see front matter. Published by Elsevier Inc.
doi:10.1016/j.jms.2003.08.007
laboratory study of the next larger hydroxyaldehyde
sugar, glyceraldehyde. Cooper et al. [2] have recently
identified the sugar 1,3-dihydroxy-2-propanone, sugar
alcohols, and sugar acids in the Murchison meteorite
which is used as the standard source for organic
compounds of extraterrestrial origin.
Glycolaldehyde was first studied in the microwave region by Marstokk and Møllendal [3] by flowing the vapor
from a heated sample through a Stark waveguide cell. Our
initial microwave studies of glyceraldehyde followed this
method but without success. A parallel study employing a
Fourier-transform microwave (FTMW) spectrometer
equipped with a heated nozzle met with better success.
Due to the possible occurrence of multiple structural
conformations, ab initio calculations were carried out
to aid in the assignment of the lowest energy forms.
264
F.J. Lovas et al. / Journal of Molecular Spectroscopy 222 (2003) 263–272
2. Experimental
Broad spectral survey scans of the rotational spectra
of glyceraldehyde and 1,3-dihydroxypropanone were
obtained using a pulsed-molecular-beam, Fabry–Perot
cavity, spectrometer of the Balle–Flygare design [4]. The
instrument used was the mini-FTMW spectrometer, as
described by Suenram et al. [5]. The first sample of
glyceraldehyde used in these studies was purchased from
Aldrich Chemical [6] with a stated purity of 95% and
was used without further purification. A second sample
of glyceraldehyde was purchased from TGI America [6]
with no purity designation. Glyceraldehyde is a solid at
room temperature and melts at 145 °C. A sample of 1,3dihydroxypropanone dimer was purchased from Alfa
Aesar [6] with a stated purity of 97% and melting point
about 80 °C. As a result of low vapor pressure at room
temperature, all samples required heating in order to
entrain them in a carrier gas at a concentration sufficient
to record their rotational spectra. This was accomplished by using a heated-reservoir nozzle installed in
the backside (atmospheric pressure side) of the integral
end flange-mirror of the mini-FTMW spectrometer [7].
Most experiments were carried out with the nozzle
temperature between 105 and 135 °C. The carrier gas
used was an 80/20% mixture of neon and helium at
150 kPa. All frequency measurements were referenced to
a rubidium frequency standard.
3. Spectral observations and analysis
3.1. Spectral measurements
For each spectral scan, 25 free induction decays were
averaged and Fourier transformed at stepped frequency
intervals of 0.5 MHz. Two data sets were obtained for
each of these scans. One data set consisted of a series of
center frequencies and peak amplitudes with one data
pair obtained from each 0.5 MHz interval. These survey
scans were convenient for making initial assignments
and for identifying different molecules and conformers
in the observed spectrum using features of the JB95
software [8]. Each assigned frequency was later refined
using a second data set that was stored at the full resolution of the instrument, typically 2 kHz.
Survey scans were carried out from 12 to 19 GHz at a
nozzle temperature of 135 °C and repetition rate of 5 Hz
using the first sample of glyceraldehyde purchased. A
second survey scan was completed later over the range
of 10–20 GHz at a nozzle temperature of 106 °C and is
shown in upper portion of Fig. 1. In order to assist in
the assignments, ab initio calculations were carried out
on a number of conformers as described in Section 4.
Beginning with the predicted rotational constants,
preliminary assignments were made to two different
Fig. 1. Upper trace shows the observed spectrum from a sample of
glyceraldehyde. Middle trace is a simulation of the assigned spectrum
for Conformer I of glyceraldehyde. Bottom trace is a simulation of the
assigned spectrum for 1,3-dihydroxy-2-propanone.
conformers, one of which was in good agreement with
the predictions. All transitions associated with the assigned spectra were then removed from the observed
spectrum using features of the software. This process
simplified the spectra considerably and made it easier to
recognize and assign transitions to the remaining molecules and/or conformers.
In the middle portion of Fig. 1 is shown the spectrum
simulated from the fitted constants of one of the assigned molecules which was identified as the lowest energy form of glyceraldehyde (Conformer I). The
assigned transition frequencies and quantum numbers
are given in Table 1. The second spectrum assigned had
only b-type transitions
and a small inertial defect,
2 . The simulated spectrum is shown
Ic –Ia –Ib ¼ 6:64 u A
in the lower portion of Fig. 1 and the assigned transitions are given in Table 2. The small inertial defect
suggested that all of the heavy atoms were in the ab
plane, assuming a structure with the empirical formula
C3 H6 O3 and four hydrogen atoms contributing to the
out-of-plane moments of inertia. Since 1,3-dihydroxy-2propanone would have a symmetric (C2v ) planar heavy
atom structure with a single dipole moment axis, we
concluded that the second form of the C3 sugar would
more likely be consistent with these structural properties. This conclusion was later supported by comparisons with the ab initio results.
Since it was not clear whether 1,3-dihydroxy-2propanone was an impurity in the original sample of
glyceraldehyde or if it was produced in the heated nozzle, we purchased a second sample of glyceraldehyde
from a difference source as well as a sample of 1,3-dihydroxy-2-propanone to establish credibility to the
F.J. Lovas et al. / Journal of Molecular Spectroscopy 222 (2003) 263–272
265
Table 1
Microwave spectrum of Conformer I glyceraldehyde, the lowest energy form
0
00
–JK;K
JK;K
Frequencya (MHz)
Obs. ) calc. (kHz)
0
00
JK;K
–JK;K
Frequencya (MHz)
Obs. ) calc. (kHz)
414 –322
202 –101
423 –414
211 –110
422 –414
524 –432
303 –211
523 –431
212 –101
634 –624
523 –515
303 –212
533 –523
413 –321
211 –101
432 –422
431 –422
331 –321
515 –423
330 –322
431 –423
532 –524
313 –212
10133.0130(20)
10347.5918(20)
10609.4420(20)
10772.9277(20)
11168.2440(20)
11399.4910(20)
11936.9400(20)
12621.4700(20)
12677.9800(20)
12699.8170(20)
12874.6560(20)
13096.2490(20)
13437.0350(20)
13788.2200(20)
13837.2910(20)
13921.0210(20)
13938.1367(20)
14186.5479(20)
14243.7500(20)
14380.8320(20)
14496.9395(20)
14743.7969(20)
14976.8135(20)
)1.5
)0.8
0.6
0.3
)0.9
1.0
0.0
1.5
)1.2
)0.8
)0.6
)0.1
)1.2
)0.2
0.7
)0.1
1.6
)0.1
)0.5
)0.6
0.9
)0.3
0.5
624 –616
633 –625
303 –202
322 –221
321 –220
734 –726
312 –211
404 –312
313 –202
221 –110
220 –110
221 –111
220 –111
514 –422
744 –734
312 –202
414 –313
542 –532
642 –634
414 –303
321 –211
515 –414
15172.6143(20)
15200.7988(20)
15426.6387(20)
15579.7363(20)
15732.6846(20)
15969.6758(20)
16133.6064(20)
16208.7891(20)
17307.2031(20)
18806.6060(20)
18845.4840(20)
19193.0510(20)
19231.9250(20)
19413.2550(20)
19507.1170(20)
19623.3030(20)
19928.9250(20)
19942.0240(20)
19994.8550(20)
21809.4893(20)
23805.2400(20)
24853.1914(20)
)0.3
0.4
0.9
0.3
)0.1
)0.2
)0.3
0.0
1.4
)0.4
1.0
2.0
)0.6
)0.6
0.7
)1.4
)0.5
)0.1
0.2
)0.1
)0.3
)0.5
a
Uncertainties shown in parentheses are Type B estimates of two standard deviations based on the resolution and signal-to-noise of the spectral
line.
Table 2
Microwave spectrum of 1,3-dihydroxy-2-propanone; the second form
of the C3 H6 O3 sugars
0
00
–JK;K
JK;K
Frequencya (MHz)
Obs. ) calc. (kHz)
321 –414
514 –505
221 –312
111 –000
615 –606
505 –414
716 –707
817 –808
212 –101
606 –515
313 –202
1019 –10010
927 –918
826 –817
432 –523
725 –716
707 –616
624 –615
414 –303
523 –514
422 –413
321 –312
220 –211
221 –212
515 –404
322 –313
10230.5000(40)
10540.6348(40)
11420.6455(40)
11536.4492(40)
11731.7598(40)
12302.5068(40)
13208.4570(40)
14995.2940(40)
15006.7725(40)
16596.6465(40)
18322.5566(40)
19546.4736(40)
19882.2109(40)
20157.6729(40)
20329.7578(40)
20592.8721(40)
20897.9053(40)
21132.7061(40)
21495.9492(40)
21720.5107(40)
22301.8613(40)
22828.0713(40)
23258.6963(40)
24198.2861(40)
24546.4023(40)
24678.6514(40)
5.1
)5.5
)2.8
)0.5
)1.3
0.8
)0.4
2.5
)1.6
0.1
0.3
)0.5
)0.4
0.4
)0.6
0.0
)0.1
)0.6
0.2
0.7
0.8
)0.4
)1.7
2.1
1.6
)0.1
a
Uncertainties shown in parentheses are Type B estimates of two
standard deviations based on the resolution and signal-to-noise of the
spectral line.
assignment of the second spectrum shown at the bottom
in Fig. 1. Survey scans of this sample showed all of
the same features observed in the original sample. We
further tested many of the unassigned lines against
conditions for optimizing signals, including nozzle
temperature and backing pressure. We found that the
observed signals from the two assigned conformers behaved similarly and exhibited constant intensity while
integrating over hundreds of nozzle pulses. In contrast,
a number of the unassigned lines showed a significant
intensity reduction during integration, which suggested
that they resulted from decomposition products. This
behavior is consistent with their expected depletion
rate at the constant 5 Hz nozzle repetition rate.
Since these sugars are carbohydrates, the logical decomposition process would be dehydration. Subtracting
H2 O from the empirical formula C3 H6 O3 gives the
empirical formula, C3 H4 O2 . Seven possible candidates
were identified with this empirical formula from a search
of the microwave literature. Rotational transitions from
one of these, trans-methyl glyoxal [9], were identified in
the spectra of both the glyceraldehyde and 1,3-dihydroxy-2-propanone samples. However, trans-methyl
glyoxal only accounted for part of the lines characterized as decomposition products.
Another way to identify possible decomposition
products of glyceraldehyde is to eliminate an OH group
and H atom from adjacent carbon atoms in the parent
structure and make a C @ C bond in the product. One of
266
F.J. Lovas et al. / Journal of Molecular Spectroscopy 222 (2003) 263–272
these structures, 2-hydroxy-2-propen-1-al, has not been
previously assigned in the microwave but has been
identified as the carrier of the second spectrum from a
decomposition product in all of our sugar spectra. As
discussed below, ab initio results confirm its identity (see
Table 3 for the assigned transitions). A third decomposition product, formic acid, was also identified in the
spectra of all samples.
In order to aid the assignment of any other conformers of glyceraldehyde, simulated spectra of the four
assigned species, glyceraldehyde, 1,3-dihydroxy-2-propanone, t-methyl glyoxal, and 2-hydroxy-2-propen-1-al,
were subtracted from the observed spectrum shown in
Fig. 1. The residual spectrum is shown at the top of
Fig. 2. Beginning with the ab initio results for the other
low energy conformers of glyceraldehyde, we were able
to assign two more conformers (Conformers II and III).
The assigned line sets are given in Tables 4 and 5, respectively. The middle trace of Fig. 2 shows the residuals
after subtraction of the spectrum from Conformer II
while the bottom trace shows the residuals after subtraction of the Conformer III spectrum. No further assignments could be made based on the small number of
remaining transitions.
It is noteworthy that the spectra from the 1,3-dihydroxy-2-propanone sample showed all the decomposition products identified in the glyceraldehyde sample,
but none of the glyceraldehyde conformers appeared in
the spectra from the ketone sugar form. It is well known
that in a moist environment there exists a dynamic
equilibrium between the aldehyde and ketone forms,
with 1,3-dihydroxy-2-propanone being the more stable
form.
Table 3
Microwave spectrum of the dehydration product 2-hydroxy-2-propen1-al
0
00
JK;K
–JK;K
Frequencya (MHz)
Obs. ) calc. (kHz)
202 –111
312 –303
111 –000
212 –111
202 –101
413 –404
321 –312
211 –110
220 –211
303 –212
212 –101
313 –212
303 –202
312 –211
313 –202
9482.2790(10)
11516.1950(20)
13343.4627(10)
13968.6687(10)
15140.6234(10)
15782.8920(10)
16139.7098(10)
16771.7137(10)
17204.4980(10)
17688.5854(10)
19627.0137(10)
20819.5625(10)
22174.9736(10)
25000.2078(10)
25305.9510(10)
)0.4
)0.8
0.0
)0.1
)0.3
0.0
0.0
0.2
0.0
0.3
0.6
0.6
)0.9
0.4
)0.4
a
Uncertainties shown in parentheses are Type B estimates of two
standard deviations based on the resolution and signal-to-noise of the
spectral line.
Fig. 2. Upper trace shows the residual spectrum after subtraction of
the four assigned species (Conformer I of glyceraldehyde, 1,3-dihydroxy-2-propanone, and decomposition products trans-methyl glyoxal
and 2-hydroxy-2-propen-1-al) from the observed spectrum in the upper
trace of Fig. 1. Middle trace shows the residuals after subtraction of
the spectrum of Conformer II glyceraldehyde from the upper trace.
Bottom trace shows the residuals after subtraction of the spectrum of
Conformer III glyceraldehyde from the middle trace.
Table 4
Microwave spectrum of Conformer II glyceraldehyde; the second
lowest energy form Erel ¼ 5:7 kJ/mol
0
00
JK;K
–JK;K
Frequency (MHz)a
Obs. ) calc. (kHz)
111 –000
615 –606
404 –313
313 –212
303 –202
312 –211
212 –101
211 –101
505 –414
404 –303
523 –514
422 –413
313 –202
321 –312
221 –212
322 –313
515 –414
606 –515
505 –404
423 –414
414 –303
514 –413
10122.5576(10)
10439.9355(10)
11037.3428(10)
11793.8828(10)
12253.1943(10)
12895.1748(10)
13888.0625(10)
14899.7080(20)
15572.1191(10)
16274.4248(20)
16569.9990(20)
17110.8408(10)
17490.2754(10)
17630.9609(10)
19070.8984(10)
19585.1846(10)
19613.7236(10)
20115.7803(10)
20244.6357(10)
20275.4482(10)
20946.9414(10)
21292.4111(10)
1.1
0.2
0.0
)0.5
0.3
0.2
0.3
)1.5
0.6
)0.2
)0.6
0.1
)0.7
0.6
)0.6
)0.1
0.5
)0.5
0.5
0.0
)0.3
)0.5
a
Uncertainties shown in parentheses are Type B estimates of two
standard deviations based on the resolution and signal-to-noise of the
spectral line.
3.2. Rotational analysis
The rotational analysis of all species employed the
Watson Ir A reduction Hamiltonian [10] with quartic
terms. The derived rotational and centrifugal distortion
F.J. Lovas et al. / Journal of Molecular Spectroscopy 222 (2003) 263–272
Table 5
Microwave spectrum of Conformer III glyceraldehyde; the fourth
lowest energy form Erel ¼ 9:8 kJ/mol
267
Table 7
Spectroscopic constants and dipole moment components for 2-hydroxy-2-propen-1-ala
0
00
JK;K
–JK;K
Frequencya (MHz)
Obs. ) calc. (kHz)
Parameter
2-Hydroxy-2-propen-1-al
211 –110
212 –101
313 –212
303 –202
322 –221
321 –220
312 –211
313 –202
404 –313
414 –313
404 –303
423 –322
432 –331
431 –330
422 –321
413 –312
515 –414
505 –404
524 –423
523 –422
514 –413
9852.555(4)
11691.408(2)
12689.695(4)
13387.862(4)
13762.655(4)
14137.345(4)
14713.300(4)
15302.808(4)
15566.700(10)
16825.058(4)
17481.642(4)
18274.440(4)
18520.309(5)
18572.074(5)
19142.083(4)
19483.019(4)
20901.194(4)
21407.326(4)
22722.761(5)
24247.457(5)
24112.353(4)
)4
0
)2
)1
)1
3
2
2
3
0
2
0
2
)1
)2
0
6
)7
)2
)1
2
A (MHz)
B (MHz)
C (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
dJ (kHz)
dK (kHz)
rb
la (C m)
la (D)
lb (C m)
lb (D)
lc (C m)
2 )
Ic –Ia –Ib (u A
10201.6867(12)
4543.3353(22)
3141.7866(20)
0.898(60)
5.99(40)
5.08(25)
0.268(15)
4.44(94)
0.58
3.823(33) 1030
1.146(10)
5.204(40) 1030
1.560(12)
0c
0.083
a
Uncertainties shown in parentheses are two standard deviations
(coverage factor k ¼ 2).
b
Unitless standard deviation of weighted fit.
c
Zero from symmetry.
a
Uncertainties shown in parentheses are Type B estimates of two
standard deviations based on the resolution and signal-to-noise of the
spectral line.
constants for the three assigned conformers of glyceraldehyde and the one form of 1,3-dihydroxy-2-propanone are given in Table 6. Similarly, the rotational
analysis of 2-hydroxy-2-propen-1-al is given in Table 7.
All fits were weighted by the inverse square of the
measurement uncertainty and the resulting unitless
standard deviation, r, is also listed in Tables 6 and 7.
Type A and B uncertainties, with coverage factor k ¼ 2
(two standard deviations), are quoted for the derived
constants and measurements, respectively [11].
3.3. Dipole moment determination
Stark effect measurements were carried out on a
number of the observed species, namely: Conformers I
and II of glyceraldehyde, 1,3-dihydroxy-2-propanone,
Table 6
Spectroscopic constants and dipole moment components for two forms of the C3 sugars
Parameter
Glyceraldehyde Conformer Ia
Glyceraldehyde Conformer IIa
Glyceraldehyde Conformer IIIa
1,3-Dihydroxy-2-propanonea; b
A (MHz)
B (MHz)
C (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
dJ (kHz)
dK (kHz)
rc
la (C m)
la (D)
lb (C m)
lb (D)
lc (C m)
lc (D)
2 )
Ic –Ia –Ib (uA
5467.77597(18)
2789.85770(16)
2403.40821(14)
1.9564(36)
)6.231(9)
13.846(16)
0.3904(14)
0.936(24)
0.44
1.531(10) 1030
0.459(3)
1.20(13) 1030
0.36(4)
6.898(27) 1030
2.068(8)
)63.30
8239.8203(9)
2219.9762(7)
1882.7530(7)
0.4896(54)
)1.424(43)
21.15(18)
0.0276(21)
1.68(37)
0.60
1.50(4) 1030
0.450(12)
4.28(12) 1030
1.283(36)
1.37(27) 1030
0.41(8)
)20.56
5826.323(23)
2632.5216(31)
1955.0362(29)
1.140(27)
)4.39(14)
0.45(24)
0.309(22)
3.4(15)
0.80
9801.2914(12)
2051.5257(7)
1735.1645(8)
0.178(6)
0.662(64)
5.38(12)
0.0287(9)
0.55(28)
0.56
0e
0e
5.894(17) 1030
1.767(5)
0e
0e
)6.64
a
d
d
d
d
d
d
)20.21
Uncertainties shown in parentheses are Type A and two standard deviations (coverage factor k ¼ 2).
Common name 1,3-dihydroxyacetone.
c
Unitless standard deviation of weighted fit.
d
Spectrum was too weak to carry out Stark effect measurements.
e
Zero due to symmetry.
b
268
F.J. Lovas et al. / Journal of Molecular Spectroscopy 222 (2003) 263–272
and 2-hydroxy-2-propen-1-al. Since the mini-FTMW
spectrometer was not equipped with Stark plates, one of
the larger units was used [12,13]. Since the 25 cm 25 cm
Stark plates do not encompass the full molecular beam
when the coaxial nozzle is used, a heated nozzle perpendicular to the direction of microwave propagation
between the mirrors is employed with the selection rule
DM ¼ 0. Calibration of the effective plate spacing is
accomplished with Stark-shifted measurements on the
J ¼ 1–0 transition of OCS. For conformer I of glyceraldehyde, the jMj ¼ 0 and 1 components of four transitions were measured, the 211 –101 , 212 –101 , 220 –110 , and
221 –111 transitions, with frequency shifts up to 1.3 MHz.
The three dipole moment components derived are given
in Table 6 with lc being the largest value. For Conformer II of glyceraldehyde, the jMj ¼ 0 and 1 of the
212 –101 transition and M ¼ 0 of the 111 –000 transition
were measured with frequency shifts up to 0.8 MHz.
Three dipole moment components were determined with
lb dominant as shown in Table 6. The spectrum of
conformer III of glyceraldehyde was too weak for Stark
effect measurements. For 1,3-dihydroxy-2-propanone
the M ¼ 0 component of the 111 –000 , jMj ¼ 0 and 1
components of the 212 –101 , and jMj ¼ 0; 1, and 2
components of the 313 –202 transitions were measured
with shifts up to 1.8 MHz. The single lb dipole moment
component is listed in Table 6. For 2-hydroxy-2-propen1-al the M ¼ 0 component of the 111 –000 , jMj ¼ 0 and 1
components of the 212 –111 , and jMj ¼ 0, and 1 components of the 202 –101 transitions were measured with
shifts up to 0.8 MHz. Since this species is planar, only
the la and lb dipole moment components were determined as shown in Table 7.
have relative energies ranging from 0 to 12 kJ/mol with
their structures shown in Fig. 3.
The geometries of the two lowest-energy forms are
shown at the top in Fig. 3. Conformer I glyceraldehyde
geometry differs from that of Conformer II mainly in the
orientation of the terminal OH group which is rotated
approximately 120° about the C3 –C4 bond into a more
compact structure. Conformers I and II both exhibit
two hydrogen bonds and combine features of the preferred conformations of both glycolaldehyde and ethylene glycol. Conformers III and IV have only a single
hydrogen bond and their structures are shown in the
middle of Fig. 3. They differ from one another mainly in
the location of the H atom on the terminal OH group,
which does not participate in the hydrogen bond. Conformer V, at the bottom of Fig. 3, also exhibits two
internal hydrogen bonds and has the most compact
structure of all conformers, but Conformer V was not
identified in the current study even though the energy is
calculated to be lower than that of Conformer III. The
structure of 1,3-dihydroxy-2-propanone is shown in
Fig. 4. It illustrates the C2v symmetry along the b-axis
and dual hydrogen bonding of the hydroxyl protons to
the carbonyl oxygen2 atom. The experimental value of
which strongly supports a C2v
Ic –Ia –Ib ¼ 6:64 u A
structure with four out-of-plane hydrogen atoms, while
the ab initio structure twists the OH groups slightly outof-plane symmetrically
which gives a C2 structure with
2 .
Ic –Ia –Ib ¼ 8:40 u A
In Table 8, the rotational constants and electric dipole moment components in the principal axis system
3.4. Ab initio calculations
Before we began the ab initio calculations, our intuition suggested that the lowest energy structure would be
a combination of the hydrogen-bonded glycolaldehyde
structure (planar heavy atoms) [3] and the gÕGa conformer of ethylene glycol [14] with a heavy atom dihedral angle of about 60° and internal hydrogen bond
between OH groups; this served as a starting point. The
search of the glyceraldehyde energy surface was carried
out using 10 starting geometries involving different orientations of the hydroxy and –CH2 O– groups relative to
the aldehyde group. These searches were carried out at
the B3LYP/6-31G(d,p) Hartree–Fock level with zeropoint energy corrections included with Gaussian 98 [15].
These 10 geometries converged to eight stable forms. A
second set of higher-level calculations of equilibrium
energies was carried out for these eight geometries using
Møller–Plesset MP2/6-311++G** theory. Three of the
conformers were found to have equilibrium energies at
least 20 kJ/mol above the lowest energy form and are not
considered further, while the remaining five conformers
Fig. 3. Theoretical structural conformations and relative energies for
the five lowest energy forms of glyceraldehyde. The dihedral angles in
Table 10, \(1,2,3,5) and \(2,3,4,6), are most helpful in distinguishing
between Conformers I, II, and III. Conformers III and IV differ mainly
in \(3,4,6,12). See Fig. 5 for atom numbers.
F.J. Lovas et al. / Journal of Molecular Spectroscopy 222 (2003) 263–272
Fig. 4. Theoretical structure of the assigned conformer of 1,3-dihydroxy-2-propanone.
are compared to those observed experimentally for the
lowest energy forms of glyceraldehyde and 1,3-dihydroxy-2-propanone. The ab initio rotational constants
269
for Conformer I glyceraldehyde all agree within 0.3%
with the observed values, while those for 1,3-dihydroxy2-propanone differ by up to 1.5%. This is still considered
excellent agreement for this level of theory. For the dipole moments calculated at the Hartree–Fock level, the
agreement is somewhat worse, but still supportive of the
identification. In Table 9 the experimental and theoretical rotational constants and dipole moments for Conformers II and III are shown. The agreement for
Conformer II is excellent with a maximum difference of
about 0.4%. The experimental rotational constants of
Conformer III are compared to the calculated values for
Conformers III and IV (see Fig. 3) whose structures
differ only in the position of the H atom on the terminal
OH group. Based on this comparison one cannot distinguish which of the theoretical forms should apply to
the experimental Conformer III. However, the experimental spectrum exhibits a-type and b-type transitions,
but c-type transitions could not be observed. Since the
Table 8
Rotational constants and dipole moment componentsa comparison with ab initio MP2/6-311++G** calculations for the two lowest energy forms of
the C3 H6 O3 sugars
Parameter
Glyceraldehyde Conformer Ib
Observed
Glyceraldehyde Conformer I
ab initio
1,3-Dihydroxy-2-propanoneb
observed
1,3-Dihydroxy-2-propanone
ab initio
A (MHz)
B (MHz)
C (MHz)
la (C m)
la (D)
lb (C m)
lb (D)
lc (C m)
lc (D)
5467.77597(18)
2789.85770(16)
2403.40821(14)
1.531(10) 1030
0.459(3)
1.20(13) 1030
0.36(4)
6.898(27) 1030
2.068(8)
5485.24 [0.3%]c
2789.53 [0.01%]
2408.75 [0.2%]
2.17 1030
0.65
0.84 1030
0.25
9.14 1030
2.74
9801.2914(12)
2051.5257(7)
1735.1645(8)
0
0
5.894(17) 1030
1.767(5)
0
0
9653.46 [1.5%]c
2039.71 [0.6%]
1732.38 [0.2%]
0
0
7.07 1030
2.12
0
0
a
Hartree–Fock level.
Uncertainties shown in parentheses are Type A and two standard deviations (coverage factor k ¼ 2).
c
Percentage difference between the observed and calculated absolute values.
b
Table 9
Rotational constant and dipole moment componenta comparison with ab initio MP2/6-311++G** calculations for several of the lowest energy forms
of glyceraldehyde
Parameter
Glyceraldehyde
Conformer IIb
observed
Glyceraldehyde
Conformer II
ab initio
Glyceraldehyde
Conformer IIIb
observed
Glyceraldehyde
Conformer III
ab initio
Glyceraldehyde
Conformer IV
ab initio
Glyceraldehyde
Conformer V
ab initio
A (MHz)
B (MHz)
C (MHz)
la (C m)
la (D)
lb (C m)
lb (D)
lc (C m)
lc (D)
Erel (kJ/mol)d
8239.8203(9)
2219.9762(7)
1882.7530(7)
1.50(4) 1030
0.450(12)
4.28(12) 1030
1.283(36)
1.37(27) 1030
0.41(8)
8240.75 [0.01%]c
2224.63 [0.21%]
1890.16 [0.39%]
1.80 1030
0.54
5.70 1030
1.71
2.07 1030
0.62
5.7
5826.334(33)
2632.5227(22)
1955.0350(41)
5778.24 [0.9%]c
2662.13 [1.1%]
1966.34 [0.6%]
10.8 1030
3.25
0.93 1030
0.28
0.43 1030
0.13
9.8
5860.52 [0.6%]c
2578.72 [1.8%]
1933.18 [1.1%]
7.51 1030
2.25
0.03 1030
0.01
6.00 1030
1.80
11.7
4364.26
3550.33
2405.23
7.51 1030
0.23
0.03 1030
0.71
6.00 1030
1.37
8.8
a
—
—
—
—
—
—
—
—
Hartree–Fock level.
b
Uncertainties shown in parentheses are Type A and two standard deviations (coverage factor k ¼ 2).
c
Percentage difference between the observed and calculated absolute values.
d
Conformer I is the lowest energy conformer where the reference energy is calculated to be )342.8018445 Hartree.
270
F.J. Lovas et al. / Journal of Molecular Spectroscopy 222 (2003) 263–272
theoretical lc dipole moment calculated for the theoretical Conformer IV is 1.8 D, this form is less likely the
one observed. In addition, the theoretical Conformer III
is calculated to be 2 kJ/mol lower in energy than Conformer IV.
Ab initio structural values and relative energies optimized at the MP2/311++G** level for the five lowest
energy forms of glyceraldehyde are listed in Table 10.
The atom numbering is shown in Fig. 5 with Conformer
II used as the reference structure. Similarly, the structures of 1,3-dihydroxy-2-propanone and 2-hydroxy-2propen-1-al are given in Table 11. The atom numbering
for 1,3-dihydroxy-2-propanone is shown in Fig. 4 and
that for 2-hydroxy-2-propen-1-al is shown in Fig. 6.
While alternate conformers for the latter two species
were not searched, they are most likely the lowest energy
forms, and the only forms of these species identified in
the laboratory spectrum.
Table 10
Ab initio MP2/311++G** structural parameters for five conformers of glyceraldehyde
Parameter
r(C2 –O1 )
r(C2 –C3 )
r(C3 –O5 )
r(C3 –C4 )
r(C4 –O6 )
r(C2 –H7 )
r(C3 –H8 )
r(C4 –H9 )
r(C4 –H10 )
r(O5 –H11 )
r(O6 –H12 )
\ð1; 2; 3Þa
\ð1; 2; 7Þ
\ð2; 3; 4Þ
\ð2; 3; 5Þ
\ð2; 3; 8Þ
\ð4; 3; 5Þ
\ð4; 3; 8Þ
\ð5; 3; 8Þ
\ð3; 4; 6Þ
\ð3; 4; 9Þ
\ð3; 4; 10Þ
\ð6; 4; 9Þ
\ð6; 4; 10Þ
\ð9; 4; 10Þ
\ð3; 5; 11Þ
\ð4; 6; 12Þ
\ð1; 2; 3; 4Þ
\ð1; 2; 3; 5Þ
\ð1; 2; 3; 8Þ
\ð7; 2; 3; 4Þ
\ð7; 2; 3; 5Þ
\ð7; 2; 3; 8Þ
\ð2; 3; 4; 6Þ
\ð2; 3; 4; 9Þ
\ð2; 3; 4; 10Þ
\ð5; 3; 4; 6Þ
\ð5; 3; 4; 9Þ
\ð5; 3; 4; 10Þ
\ð8; 3; 4; 6Þ
\ð8; 3; 4; 9Þ
\ð8; 3; 4; 10Þ
\ð2; 3; 5; 11Þ
\ð4; 3; 5; 11Þ
\ð8; 3; 5; 11Þ
\ð3; 4; 6; 12Þ
\ð9; 4; 6; 12Þ
\ð10; 4; 6; 12Þ
a
Conformer I
1.216 A
1.516 A
1.411 A
1.524 A
1.415 A
1.106 A
1.103 A
1.097 A
1.094 A
0.969 A
0.964 A
121.71°
121.85°
110.67°
110.49°
107.02°
108.60°
109.03°
111.04°
110.69°
108.59°
110.26°
111.63°
107.02°
108.63°
105.55°
105.71°
127.66°
7.34°
)113.66°
)53.32°
)173.64°
65.35°
)59.20°
177.92°
59.02°
62.23°
)60.64°
)179.54°
)176.65°
60.47°
)58.43°
)13.14°
)134.69°
105.44°
)56.10°
64.99°
)176.27°
Conformer II
Conformer III
Conformer IV
Conformer V
1.217 A
1.510 A
1.412 A
1.529 A
1.413 A
1.106 A
1.100 A
1.094 A
1.099 A
0.969 A
0.964 A
1.219 A
1.516 A
1.406 A
1.527 A
1.424 A
1.102 A
1.101 A
1.097 A
1.096 A
0.969 A
0.960 A
1.219 A
1.515 A
1.405 A
1.533 A
1.422 A
1.102 A
1.103 A
1.093 A
1.095 A
0.969 A
0.961 A
1.218 A
1.525 A
1.421 A
1.521 A
1.425 A
1.107 A
1.098 A
1.091 A
1.095 A
0.967 A
0.964 A
121.75°
121.58°
110.81°
111.26°
108.29°
108.76°
108.25°
109.42°
110.36°
110.62°
108.68°
106.92°
111.20°
109.06°
105.96°
105.67°
118.05°
)3.08°
)113.66°
)61.21°
177.65°
57.36°
179.75°
61.63°
)58.07°
)57.65°
)175.78°
64.53°
61.15°
)56.97°
)176.67°
1.84°
)120.48°
121.46°
51.95°
172.32°
)68.74°
121.03°
122.11°
110.31°
111.10°
107.38°
109.06°
108.00°
110.93°
106.88°
109.15°
108.09°
111.77°
112.07°
108.78°
105.43°
107.92°
122.32°
1.25°
)120.22°
)57.51°
)178.58°
59.95°
61.70°
)59.34°
)177.50°
)176.02°
62.93°
)55.22°
)55.38°
)176.42°
62.93°
)4.01°
)125.82°
115.35°
172.00°
)68.63°
53.77°
For the angles, the numbers in parentheses are the atom numbers from Fig. 5.
121.18°
122.09°
110.22°
111.14°
107.06°
109.35°
108.31°
110.70°
111.38°
109.62°
108.40°
106.86°
112.40°
108.10°
105.54°
107.61°
124.84°
3.45°
)117.56°
)54.58°
)175.97°
63.02°
63.92°
)54.13°
)171.92°
)173.62°
68.33°
)49.45°
)52.90°
)170.95°
71.26°
)6.70°
)128.60°
112.14°
76.61°
)163.69°
)45.27°
123.80°
121.44°
111.85°
108.40°
108.10°
111.31°
110.23°
106.77°
109.99°
109.05°
110.42°
106.40°
111.58°
109.30°
105.11°
106.17°
)7.50
)130.59°
114.03°
170.41°
47.32°
)68.06°
)64.93°
178.75°
58.64°
56.49°
)59.83°
)179.94°
174.78°
58.46°
)61.65°
83.10°
)40.32°
)160.66°
76.94°
)165.09°
)45.95°
F.J. Lovas et al. / Journal of Molecular Spectroscopy 222 (2003) 263–272
Fig. 5. Atom numbering for structural parameters of the conformers of
glyceraldehyde using Conformer II as a reference structure.
4. Discussion
The agreement in rotational constants for each of the
experimentally assigned conformers of glyceraldehyde,
1,3-dihydroxy-2-propanone and 2-hydroxy-2-propen-1al with those derived from the theoretical structures is
excellent. The fact that each of these structures have one
or two hydrogen bonds, show the preferential stabil-
271
Fig. 6. Atom numbering for structural parameters of the conformers of
2-hydroxy-2-propen-1-al.
ization of the intramolecular hydrogen bond. For 1,3dihydroxy-2-propanone the C2 theoretical structure
does not agree as well as expected with the experimental
evidence for a C2v structure. Even calculations of the
B3LYP type with the same basis set as the MP2 calculations for a planar heavy atom conformation did not
confirm this structure as a minimum, since one negative
vibrational frequency was found. A higher level of theory appears necessary in this case.
Table 11
Ab initio MP2/311++G** structural parameters for 1,3-dihydroxy-2-propanone and 2-hydroxy-2-propen-1-al
Parameter
r(C2 –O1 )
r(C2 –C3 ); r(C2 –C4 )
r(C3 –O6 ); r(C4 –O5 )
r(C3 –H7 ); r(C4 –H10 )
r(C3 –H8 ); r(C4 –H9 )
r(O5 –H11 ); r(O6 –H12 )
\ð1; 2; 3Þ; \ð1; 2; 4Þa
\ð3; 2; 4Þ
\ð2; 3; 6Þ; \ð2; 4; 5Þ
\ð2; 3; 7Þ; \ð2; 4; 10Þ
\ð2; 3; 8Þ; \ð5; 4; 9Þ
\ð6; 3; 7Þ; \ð5; 4; 10Þ
\ð6; 3; 8Þ; \ð5; 4; 9Þ
\ð7; 3; 8Þ; \ð9; 4; 10Þ
\ð4; 5; 11Þ; \ð3; 6; 12Þ
\ð1; 2; 3; 6Þ; \ð1; 2; 4; 5Þ
\ð1; 2; 3; 7Þ; \ð1; 2; 4; 10Þ
\ð1; 2; 3; 8Þ; \ð1; 2; 4; 9Þ
\ð4; 2; 3; 6Þ; \ð3; 2; 4; 5Þ
\ð4; 2; 3; 7Þ; \ð3; 2; 4; 10Þ
\ð4; 2; 3; 8Þ; \ð3; 2; 4; 9Þ
\ð2; 3; 6; 12Þ; \ð2; 4; 5; 11Þ
\ð7; 3; 6; 12Þ; \ð10; 4; 5; 11Þ
\ð8; 3; 6; 12Þ; \ð9; 4; 5; 11Þ
1,3-Dihydroxy-2-propanone
1.221 A
1.515 A
1.404 A
1.102 A
1.095 A
0.966 A
120.80°
118.40°
112.13°
106.41°
109.80°
111.80°
109.06°
107.52°
105.76°
16.53°
)105.99°
137.94°
)163.46°
74.02°
)42.05°
)25.01°
94.40°
)146.84°
Parameter
r(C1 –C2 )
r(C1 –H6 )
r(C1 –H7 )
r(C2 –O3 )
r(C2 –C4 )
r(O3 –H8 )
r(C4 –O5 )
r(C4 –H9 )
\ð2; 1; 6Þ
\ð2; 1; 7Þ
\ð6; 1; 7Þ
\ð1; 2; 3Þ
\ð1; 2; 4Þ
\ð3; 2; 4Þ
\ð2; 3; 8Þ
\ð2; 4; 5Þ
\ð2; 4; 9Þ
\ð5; 4; 9Þ
2-Hydroxy-2-propen-1-al
1.347 A
1.084 A
1.083 A
1.351 A
1.480 A
0.970 A
1.222 A
1.104 A
118.88°
121.79°
119.33°
123.77°
120.74°
115.50°
105.62°
120.92°
116.69°
122.39°
a
For the angles, the numbers in parentheses are the atom numbers from Fig. 4 for 1,3-dihydroxy-2-propanone and Fig. 6 for 2-hydroxy-2propen-1-al.
272
F.J. Lovas et al. / Journal of Molecular Spectroscopy 222 (2003) 263–272
Conformer V was not identified in the current study
even though the energy is calculated to be 1 kJ/mol
lower than that of Conformer III. There could be several
explanations for this. First, in previous work on the
straight chain 1-alkene molecules up to C12 , it has been
shown that the ability of ab initio theory to predict energy level ordering among conformers with nearly the
same energy is not always possible [16,17]. Thus, in reality, the energy of Conformer V may lie above the true
energy of Conformer III and the spectrum may be too
weak to observe. For conformer V, the dipole moment
components are calculated to be a factor of three smaller
than la of conformer III, which reduces the intensity of
the spectrum and making assignment more difficult. It is
also possible that even if the energy of Conformer V is
lower than that of Conformer III, the barrier to conversion to either Conformer I or Conformer II may be
quite low and thus in the cold molecular beam environment, it may relax down to Conformer I or Conformer II. It is often found that if this barrier to
conversion is below about 400 cm1 , the conformer will
be cooled out in the molecular beam [18]. This was
found to be the case for one of the higher energy conformers of 1-pentene [19].
5. Summary
The laboratory microwave spectra and theoretical
structures for three conformers of the sugar glyceraldehyde and one conformer of the alternate form 1,3-dihydroxy-2-propanone are reported for the first time.
The rotational constants for all four species derived
from the theoretical structures agree with the values
derived from the spectral analysis to better than 1.5%.
Three decomposition products have been identified:
formic acid, trans-methyl glyoxal and 2-hydroxy-2-propen-1-al. The 2-hydroxy-2-propen-1-al species, a dehydration product, is also observed for the first time and
its theoretical molecular constants are also in excellent
agreement with the laboratory data.
Acknowledgments
Anne Horn is thanked for her assistance. The Research Council of Norway (Programme for Supercom-
puting) is thanked for a grant of computer time at the
IBM RS6000 cluster at the University of Oslo.
References
[1] J.M. Hollis, F.J. Lovas, P.R. Jewell, Astrophys. J. (Lett.) 540
(2000) L107–L110.
[2] G. Cooper, N. Kimmich, W. Bellsle, J. Sarinanna, K. Brabham,
L. Garrel, Nature 414 (2001) 879–883.
[3] K.-M. Marstokk, H. Møllendal, J. Mol. Struct. 5 (1970) 205–213.
[4] T.J. Balle, W.H. Flygare, Rev. Sci. Instrum. 52 (1981) 33–45.
[5] R.D. Suenram, J.U. Grabow, A. Zuban, I. Leonov, Rev. Sci.
Instrum. 70 (1999) 2127–2135.
[6] In order to adequately describe certain chemicals, the commercial
source is used. The use of these names does not imply an
endorsement by the NIST in any way..
[7] R.D. Suenram, F.J. Lovas, D.F. Plusquellic, A. Lesarri, Y.
Kawashima, J.O. Jensen, A.C. Samuels, J. Mol. Spectrosc. 211
(2002) 110–118.
[8] D.F. Plusquellic, R.D. Suenram, B. Mate, J.O. Jensen, A.C.
Samuels, J. Chem. Phys. 115 (2001) 3057–3067.
[9] C.E. Dyllick-Brenzinger, A. Bauder, Chem. Phys. 30 (1978) 147–
153.
[10] J.K.G. Watson, in: J.R. Durig (Ed.), Vibrational Spectra and
Structure, vol. 6, Elsevier, Amsterdam, 1977, pp. 1–88.
[11] B.N. Taylor, C.E. Kuyatt, NIST Technical Note 1297, U.S.
Government Printing Office, 1994.
[12] F.J. Lovas, R.D. Suenram, J. Chem. Phys. 87 (1987) 2010–2020.
[13] R.D. Suenram, F.J. Lovas, W. Pereyra, G.T. Fraser, A.R. Hight
Walker, J. Mol. Spectrosc. 181 (1995) 67–77.
[14] D. Christen, L.H. Coudert, J.A. Larsson, D. Cremer, J. Mol.
Spectrosc. 205 (2001) 185–196.
[15] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A.
Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr.,
R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D.
Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V.
Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C.
Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q.
Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, B.B. Stefanov, G.
Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A.
Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B.
Johnson, W. Chen, M.W. Wong, J.L. Andres, C. Gonzalez, M.
Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian 98, Revision
A.6, Gaussian, Inc., Pittsburgh, PA, 1998.
[16] G.T. Fraser, R.D. Suenram, C.L. Lugez, J. Phys. Chem. 104
(2000) 1141–1146.
[17] G.T. Fraser, R.D. Suenram, C.L. Lugez, J. Phys. Chem. 105
(2001) 9859–9864.
[18] R.S. Ruoff, T.D. Klots, T. Emilsson, H.S. Gutowsky, J. Chem.
Phys. 93 (1990) 3142–3150.
[19] G.T. Fraser, L.-H. Xu, R.D. Suenram, C.L. Lugez, J. Chem.
Phys. 112 (2000) 6209–6217.