Malonaldehyde 1 possesses a very short intramolecular hydrogen bond. Its potential energy surface has two local minima (the two mirror image hydrogen-bonded structures) separated by a C2v transition state. Schaefer reports a high-level computational study for the search for even shorter hydrogen bonds that might even lead to a single well on the PES.1

1
2
3
4
5
6
7
8

R1
H
H
H
H
NH2
OCH3
C(CH3)3
NH2

R2
H
CN
NO2
BH2
H
H
H
NO2

The hydrogen bond distance is characterized by the non-bonding separation between the two oxygen atoms. Table 1 shows the O…O distance for a number of substituted malonaldehydes computed at B3LYP/DZP++. Electron withdrawing groups on C2 reduce the O..O distance (see trend in 1 → 4). Electron donating groups on C1 and C3 also reduce this distance (see 5 and 6). Bulky substituents on the terminal carbons also reduce the O…O distance (see 7). Combining all of these substituent effects in 8 leads to the very short O…O distance of 2.380 Å.

Table 1. Distance (Å) between the two oxygen atoms and the barrier for hydrogen transfer of substituted malonaldehydes .1

Compound

r(O…O)

ΔEa

ΔEb

1

2.546

3.92

1.54

2

2.526

3.56

1.24

3

2.521

3.34

1.04

4

2.499

2.62

0.40

5

2.474

2.02

-0.06

6

2.498

 

 

7

2.466

 

 

8

2.380

0.43

-0.78

aFocal point energy. bFocal point energy and corrected for zero-point vibrational energy.

A shorter O…O distance might imply a smaller barrier for hydrogen transfer between the two oxygens. The structures of 8 and the transition state for its hydrogen transfer are shown in Figure 1. The energies of a number of substituted malonaldehydes were computed using the focal point method, and the barriers for hydrogen transfer are listed in Table 1. There is a nice correlation between the O…O distance and the barrier height. The barrier for 8 is quite small, suggesting that with some bulkier substituents, the barrier might vanish altogether, leaving only a symmetric structure. In fact, the barrier appears to vanish when zero-point vibrational energies are included.

8

8TS

Figure 1. B3LYP/DZP++ optimized geometries of 8 and the transition state for hydrogen transfer 8TS.1

References

(1) Hargis, J. C.; Evangelista, F. A.; Ingels, J. B.; Schaefer, H. F., "Short Intramolecular Hydrogen Bonds: Derivatives of Malonaldehyde with Symmetrical Substituents," J. Am. Chem. Soc., 2008, 130, 17471-17478, DOI: 10.1021/ja8060672.

InChIs

1: InChI=1/C3H4O2/c4-2-1-3-5/h1-4H/b2-1-
InChIKey=GMSHJLUJOABYOM-UPHRSURJBI

2: InChI=1/C4H3NO2/c5-1-4(2-6)3-7/h2-3,6H/b4-2-
InChIKey=BHYIQMFSOGUTRT-RQOWECAXBC

3: InChI=1/C3H3NO4/c5-1-3(2-6)4(7)8/h1-2,5H/b3-1+
InChIKey=JBBHDCMVSJADCE-HNQUOIGGBS

4: InChI=1/C3H5BO2/c4-3(1-5)2-6/h1-2,5H,4H2/b3-1+
InChIKey=IQNKNZSFMBIPBI-HNQUOIGGBX

5: InChI=1/C3H6N2O2/c4-2(6)1-3(5)7/h1,6H,4H2,(H2,5,7)/b2-1-/f/h5H2
InChIKey=AOZIOAJNRYLOAH-KRHGAQEYDI

6: InChI=1/C5H8O4/c1-8-4(6)3-5(7)9-2/h3,6H,1-2H3/b4-3+
InChIKey=BYYYYPBUMVENKB-ONEGZZNKBI

7: InChI=1/C11H20O2/c1-10(2,3)8(12)7-9(13)11(4,5)6/h7,12H,1-6H3/b8-7-
InChIKey=SOZFXLUMSLXZFW-FPLPWBNLBX

8: InChI=1/C3H5N3O4/c4-2(7)1(3(5)8)6(9)10/h7H,4H2,(H2,5,8)/b2-1+/f/h5H2
InChIKey=IHYUFGCOUITNJP-CHFMFTGODK